If the file is a media file, using the Preview button can help decide whether to continue.

If it's a program file, the OS X "programs" can be compared for file size to see the ranges offered. I wouldn't risk an OS X OS disk, since they are just not worth the risk and are pretty cheap anyway. For other programs, go to the home site of the program and see if they offer a demo first.

Once you know what the "real thing" is, you might then be in a better position to evaluate the stuff that shows in the wild.

btw--.dmg files are really the only ones worth looking at, since those are primarily for OS X. The .zip files could be for Windows, and the .sit files are most likely for OS 9