By the Hash values (SHA1/MD5/CRC). If you don't care whether it's an authentic ISO image, your can look under the [Channel] heading in \sources\ei.cfg. BTW, it doesn't matter which you have. They're both identical with it exception of the ei.cfg file and the resulting installation will be the same for both.
Shouldve said on the site you got it from unless it was somewhere dodgey. The name of the image should say too.