Guess I'm not sure what you mean, in the store it clearly says what version [xp,vx,vxa,mv] that its made for.
But for example http://www.rpgmakerweb.com/a/vexed-enigma/pop-horror-city
as you can see it says "optimized for vxa and mv" that's how you can tell and like I mentioned above if you have ace there are ways to convert but you have to pay attention to pack creators terms I believe resizing etc would go down as modifying and I don't think many pack makers allow such but I could be wrong.