You'll need shell level access (I haven't tested it in a browser, but suspect you'll either hit memory_limit or max_execution_time).
Download the PHP file, and extract with tar
tar xvzf J3_Compat.tar.gz
Change directory to the one containing your component (I'm assuming you're doing this on a development machine, but it should also work on a live install)
Copy the file to the current directory
cp ~/Downloads/J3_Compat.php ./
Now run it
The script will check every PHP and/or XML file in this directory and it's subdirectories. The output isn't that helpful except to tell you which file, and what it thinks it's found. Where the match involves a class (e.g. JRequest::clean) it's worth checking that the function of variable for that class is actually being used with that class. It may be that the file contains JRequest and contains clean() but clean() has been used with a completely different class. Once you're sure, head over to the Joomla Documentation to see what you need to do/change.
Tidying up afterwards is nice and easy
The code comes with no warranty what-so-ever, I'm pretty certain it won't trash your machine but can't accept responsibility if it does. I also can't accept any responsibility if it slows your machine down whilst running, it's perhaps not as efficient as it could be but it works well. Any bugs will probably be answered with "It works on my machine!" but please do send them through so I can fix them.
Fixed the broken links to the file, sorry! Had uploaded the archives in a different format and forgot to update the links when I changed it!
Also, it's occurred to me that whilst this should be obvious, I should say: Just because this script doesn't find anything doesn't guarantee that your extension will work with Joomla 3.0!