How to check for Carpenter Ants?

[Lynn Harrison]

I just looked under a friend's deck which needs to be replaced. I know there are carpenter ants there, as they have seen them in the house, but I see no signs of them under the deck. No ants- didn't see any piles of poop. Didn't see any grooved places on the wood. Granted I had a weak flashlight and did not poke with anything but my fingers... The deck is redwood- do they not eat old rotten redwood?