spirit_66: Analyzing your played games with GNU is indeed an efficient way to catch the cheaters, but since it is not that rare even for casual players to play one BG game close to perfection you need to be a little more quantitative :

>BG games are analysed by GNU

How many games ?

>and one can see that ones opponent didn't make any mistake

Didn't make any mistake greater than what threshold ? GNU doesn't color mistakes lower than a certain threshold.

> and what if this happens again and again???

How many times ?

E.g. I would certainly take an aggregate error rate below 1 (Snowie) or 2 (GNU) based on two or three consecutive seven-pointers as an evidence for bot cheating.