How to Check for Plagiarism Using a Text Comparison Tool

So here’s a secret that most plagiarism detection services don’t want you to know: at their core, they’re just doing text comparison. Really fancy text comparison with massive databases, sure. But the fundamental mechanic? It’s a diff.

I discovered this when a professor friend asked me to quickly check if two student submissions looked suspiciously similar. I didn’t have access to Turnitin, but I did have a text comparison tool. Five minutes later, we found that 40% of both papers were identical, word for word. Sometimes the simplest solution works.

How Basic Plagiarism Detection Actually Works

Let’s demystify this. When you submit a paper to something like Turnitin or Grammarly’s plagiarism checker, here’s what happens behind the scenes:

Text fingerprinting: Your document gets broken into smaller chunks (usually sentences or phrases)
Database comparison: Those chunks get matched against billions of sources
Similarity scoring: The tool calculates what percentage matches existing content
Report generation: You get a pretty report highlighting the matches

But what if you don’t need to check against the entire internet? What if you just need to compare two documents?

When a Simple Diff Tool Is All You Need

A text comparison tool works perfectly for these scenarios:

Comparing two student submissions

Teachers often need to check if students copied from each other, not from the internet. Paste both papers side by side, and identical passages light up immediately. I’ve seen instructors catch cheating in under a minute this way.

Verifying original vs. submitted work

Got an employee who claims they wrote something original? Compare it to the source you suspect they copied from. The diff will show exactly what was lifted and what was changed.

Checking your own work against sources

Before submitting an article, compare your draft against the research papers you referenced. This catches accidental close paraphrasing that could flag as plagiarism later.

Contract and legal document review

Lawyers use diff tools constantly to compare document versions. When checking if someone plagiarized terms and conditions or legal language, a side-by-side comparison reveals everything.

A Step-by-Step Guide to Checking Plagiarism with a Diff Tool

Here’s my quick method:

Step 1: Get Your Text Ready

Copy the suspected plagiarized content into one panel. Copy the original source (if you have it) into the other panel. Most diff tools accept plain text or can import common file formats like PDF, DOCX, and TXT.

Step 2: Run the Comparison

Hit compare. The tool will highlight:

Identical text (exact matches)
Similar text (minor word changes)
Different text (original content)

Step 3: Interpret the Results

Here’s where judgment comes in. Not all matches are plagiarism:

Common phrases like “according to research” will match across documents
Quoted material with proper citations is fine
Technical terms and industry jargon naturally repeat

Look for sustained passages of identical text. A matching sentence here and there might be coincidence. Three consecutive matching paragraphs? That’s a problem.

Step 4: Calculate Similarity

Most diff tools show a similarity percentage. For academic work, here’s a rough guide:

0-15%: Generally acceptable (common phrases, properly quoted material)
15-25%: Worth investigating
25%+: Likely contains significant copied content

The Honest Limitations

Look, I’m not going to pretend a diff tool replaces Turnitin. Here’s what it can’t do:

It requires a known source

This is the big one. A diff tool compares two documents. It won’t tell you if something was copied from a random website you’ve never seen. For that, you need a service with a massive database.

It misses paraphrased plagiarism

If someone rewrote a source in different words but kept the same ideas and structure, a diff tool won’t catch it. Advanced plagiarism checkers use AI to detect this. Diff tools just see different text.

It won’t catch mosaic plagiarism

This is when someone takes bits from many sources and stitches them together. Each individual piece might be too small to flag, but the whole document is still plagiarized.

Language barriers

Most diff tools work best with English. If you’re checking content in other languages, accuracy may vary.

When to Use a Dedicated Plagiarism Service

You need the heavy-duty tools when:

You’re checking against the entire internet (Turnitin, Grammarly, Copyscape)
You need to detect AI-generated content (GPTZero, Originality.ai)
You’re handling high-stakes academic submissions
You need audit trails for institutional compliance
You’re checking content in multiple languages

Services like Grammarly’s plagiarism checker compare against over 16 billion web pages. Turnitin has decades of student submissions in their database. That kind of coverage requires their infrastructure.

The Practical Middle Ground

Here’s what I actually recommend to teachers and content managers:

First pass: Use a diff tool

If you suspect two specific documents are similar, start here. It’s instant, free, and private. No need to upload sensitive content to third-party servers.

Second pass: Use a plagiarism checker

If the diff tool comes up clean but you still suspect copying, then run it through Turnitin or a similar service.

For ongoing monitoring: Set up automated checks

If you’re running a publication or managing student submissions at scale, integrate a plagiarism API into your workflow.

Privacy Considerations

One thing people forget: when you upload content to online plagiarism checkers, that content often gets added to their database. Some services explicitly store submitted work to improve their detection capabilities.

Text comparison tools that run locally or in-browser don’t have this issue. Your content stays on your machine. For sensitive documents, this matters a lot.

Quick Tips for Better Results

Clean up formatting first

Remove headers, footers, page numbers, and other noise. They can throw off similarity percentages.

Compare similar lengths

If one document is 500 words and the other is 5,000, the similarity percentage will be misleadingly low.

Check multiple sources

Plagiarizers often copy from several places. If your diff tool supports it, compare against multiple suspected sources.

Save your comparisons

If you’re building a case for academic misconduct, export the comparison results. Screenshots work, but most tools let you save or print reports.

The Bottom Line

A text comparison tool won’t replace comprehensive plagiarism detection services. But it’s surprisingly effective for quick checks, comparing known documents, and investigating suspicious similarities. It’s fast, it’s private, and it’s free.

Think of it as the difference between a flashlight and a searchlight. The flashlight won’t illuminate everything, but when you know where to point it, it does the job just fine.

Next time you need to check if two documents are too similar, try a diff tool first. You might be surprised how often it’s all you need.

Need to compare documents for potential plagiarism? Our text comparison tool makes it easy to spot identical passages between any two documents. Side-by-side highlighting, similarity scoring, and instant results—no signup required.

References

Plagiarism Checker - Grammarly - Grammarly’s AI-powered plagiarism detection service
Comparative analysis of text-based plagiarism detection techniques - PMC/NIH research on plagiarism detection methods
Technology solutions for the global education community - Turnitin - Leading academic plagiarism detection platform
AI and Plagiarism Detection Software: A Comparative Guide - University of Chicago Academic Technology Solutions
Text Compare Tool - Originality.AI - Text comparison for plagiarism checking