look at the games on each card?
try to save a game to the memory card using a real ps2 and a ps2 game?
look up the markings if there are any at all?
There a lot options....the easiest and fastest way is if you have access to a real PS2, or if you have one of those USB Memory Card Adapters. Atleast you could like at the size of the memory card (PS2 = 8 megs, PSX=128k [ i think]....but third party ones exist that are larger. But i've never seen an 8 meg PSX memory card, nor will there be a PS2 memory card smaller than 8 megs)