This is a rule I made for my homebrew, but it should work well in S&W, BFRPG, etc.
If characters or monsters are aware of an enemy and have time to prepare, they can ambush their foes. The targets have a 1 in 1d6 chance of detecting an ambush. If they are actively searching for an ambush, they can detect it on a 1-2 on 1d6. Ambushing creatures do not have to check for surprise, and their foes are surprised on a 1-3 on 1d6.