I have a set of sti heads or so i was told so how do i know if they are ? what is the difference ?

 

First off what year?

But if they are either version 3/4 or 5/6

Then they will be of the under bucket shim type. This means that the buckets under the cam will have the shim underneath them. If they are not STI heads the shim sits on top the bucket. If this is the case you'll be able to see it ontop of the bucket and it will be roughly the size of an old 10 pence peice.