Pr0zac the best way to identify them is just to look at a lot of Pachanoi and Peruvianus and Bridgesii until you become familiar with their appearance.  Then when you see a different type of cactus something just won't look right about it.  There are several species that look similar and often get confused.  The main ones are Stetsonia coryne, Myrtillocactus geometrizans, Browningia hertlingiana, and Polaskia chichipe.

Like DFZ said, Keeper Trout has some great resources on his website troutsnotes.com  My favorite resource of his is this one:

[URL unfurl="true"]https://troutsnotes.com/pdf/SanPedro_2006_with_pachanoi_pachanot_addendum.pdf[/URL]

It is packed with tons of knowledge about these cacti.  And there are many photos that you can look at to see what these species look like.  That book was a huge help to me when I was starting to collect these cacti.