When you look at impact toys, you can initially test them this way, according to my top: use the inside of your forearm. No, it doesn't tell you everything you need to know to use the toy well, but you can get an idea as to the type of impact-sting, thud-and the level of pain in can inflict. Then you can decide if it matches your tastes.
As an example, he had me test all of his toys this way, in front of him. I found a few that I really like--I go for sting-y--and a couple that I wasn't so crazy about-the ping-pong paddle still scares me, as it had a pretty solid thud, even when tested lightly, and the felt flogger is just evil: swing as hard as he might, he cannot cause me pain with it! I feel it make contact, but it never hurts. With the way I feel about pain, and just the amount he has pushed me farther than I thought I could go, both in terms of intensity and duration, for him to hit me over and over and never hurt me is real torture!
Sorry if I got too long-winded, but I hope this helps!