1) There are cvar checks - when I cheat some server-side plugins kick me for cvar changes, at least in TF2. One funny plugin lies and says "VAC ban detected" lol.
2) Read the VAC forums - it is possible, through rare because of delayed bans. Visit the vac forums or cheating sites. Some sites hide the fact that they are detected. Enhancedaim, artificial aiming, private hax, etc. are all open and honest about being detected.