how do u tell if its a 4cyl t-5 or v-8 t-5

Externally, other that breaking down the tag numbers I don't think there's a fast, easy way to tell.

4 cyl T5 has a smaller input and different gear ratios. You'll probably have to make a judgement call if you trust the guy's telling you the truth.
 
4 cyl T5 will live a short life behind a V8. I know a guy that got a bunch of passes at the drag strip with one (40 or 50), but it doesn't have the strength to live too long behind a V8.
 
Got an old beat up 4 banger T5 too and sticking with it... Busted my engine's timing chain (this is a non-roller block) before the tranny lol I also sort of busted an older 7.5 rear end from a GT that was once locked, until my foot hit the accelerator. :D