I tend to look for a readme.txt in the root of the zip file. That used to be an informal standard which many developers followed. Lately after being named to anything and placed anywhere, I search for *.txt.

The issue with the source zip is that you can't just compile it and expect to have a working site without manual adjustments. During the site installation, there's a divide by zero error. I posted about this issue. The readme.txt doesn't explain what to do if you intend to use the source zip for a running site. The fix as explained elsewhere is to modify the projects to be in release mode (should be in that mode in the first place) or unzip the install zip on top of the the source zip. These little nuggets should be in the readme.txt file.