I've developped a confirmation method to be 100% sure of the identity of the mystery minifig. I was 16 out of 21 on he 1st series, then 16/16 for series 2-4 with this technique. You would benefit from some basic knowledge/experience of shapes/ridges/notches etc from years of playing with LEGOs.
I usually buy one package, open it in my car and go back in the store with the picture sheet and put it in front of me to help with visualizing what your feeling/touching. You can also use pictures on the box itself.
Ok, for example, the dwarf in Series 5, I'll start by feeling for the head piece (normally the biggest/bulkiest piece other than the body/legs). Once I have identified the helmet, to be 100% sure I have the dwarf, I'll feel for a confirmation piece. I know there should be a beard so I feel for that too until I get it. And so on for all the other minifigs. Maximum 45-60 min depending on your experience and if it's a new box.
Good luck and have fun! Hope this helps!