So, you you wanna check, that user is NOT a robot - you can use captcha or something, that robots can't do well. But, if you need check, that user is a robot?
For example, we create some multiplayer game, were players will programming robots. [this just example, imagine, that we can't run user code on server]. In this case, user can create some UI for a this game and play game by himself. This is not good for a game balance and other game things.
So, we must check, that user is a robot!
We can send request like "calculate MD5 summ of this or that", but programmer can automatize only this part, and continue to play himself with UI.
So,how can we verify this?
ps. sorry for bad english, but i hope, you understand what I mean.