Unless you are being told otherwise, chances are you have Psilocybe Cubensis mushrooms. Unfortunately we don't allow IDing here, you'd have to take that to The Shroomery, but the more potent species are rather petite looking when dried. Just round up some pictures of dried Cubensis and judge the size and thickness of the stems as well as the properties of the caps.
Considering the bigger potency and cultivation challenges of other types, it is unlikely that you will get them for a regular prices just sold as random 'shrooms, but he has a point - if you do happen to get others it is certainly best to find out first.
That said, all mushrooms - even within the same subspecies or strain, can vary significantly in potency because of growing parameters and general genetic variency. In fact, this is true for mostly all natural products, and it can be a disadvantage to synthetic products unless you homogenize your batch by pulverizing and mixing thoroughly.