They won't catch you if you're smart about it. Not logging in for < 24 hours and suddenly gaining ALL achievements for some games throws up some red flags. Plus, most people don't bother to reorganize their achievements to look legitimate (i.e. in Halo if you have the achievement for finishing the game before getting the one for completing the 2nd level) nor do they bother to set specific believable times for their achievements, such as getting the 1st mission achievement and 7~ hours later receiving the game-completed achievement.
You also have to watch out for the online achievements (i.e. in CoD, getting a prestige achievement before even reaching prestige threshold, or even having the achievement listed on a date and time when you were actually offline, thus making that impossible).