I'm impatient, so I often will test 6 smellies at once. I just swipe increasingly further up my arms. One to the back of the hand, one to the back of the forearm, one further up on the forearm. And I agree with everyone else: I let them sit and sniffy them wet, just-dried, and then after about two hours and longer if I can. BPAL and a lot of the other perfume oil-tailers usually are layered like that, where there's multiple stages that may smell different.

And I can never get enough off to re-apply once I'm wearing something, so I just go until I run out of skin. :D Then I'll usually give things a second smell all by their own, swiped on one wrist and then rubbed together, so I can tell what two different amounts will smell like.