Noob question. By looking under the hood, can you tell what % pulley is installed?

 

Not unless it is marked.

 

So all pulleys are same size?

 

Are you kidding? You say you a have the 15% in your signature.....? Do you mind if I ask why you are asking?
The most common sizes are 15%, 16%, 17%, and 19%....
All but the earliest pulleys are market have a few screws/bolts present on the end when you look at it and (usually) a stamped reduction # on it, and the brand pulley in most cases(they are usually stainless steel too) the oem's are black metal...and are hot pressed on (no bolts)...a few of the earliest pulleys on MINI's had hot pressed reduction pulleys...but they are kinda rare....and are unlikely to be on any car newer than an 04' (matbe even earlier)I think.

 

Go out look on the end of the supercharger, try to get a brand and take a diameter measurement and report back. Of course because these things sit around a shop with other pulleys before getting shipped out, checking for a % marking on the pulley might not hurt either.

 

A friend of my trying to buy a modded MINI and he wants to know if there is a way to verify if what the seller told him is correct....

 

Gimme the brand and OD, I'll tell you what it is.