People have mentioned to leave water out for them to find. Also a plastic soda bottle works Depending on size of snake. Put a mouse inside or a couple and if it goes in to eat it should be too big to exit.
Another trick is to dust doorways with flour so you can find a trace if they go through it. Get shopping bags and put that through the house lining your floorboards, this one take patiences because you have to listen out for the crackling sound of the snake along the bags. Your snake is going to be more active at night, you can loose a few nights sleep.
As bfckarlos said, doing a search on this forum you can get more ideas.. Good luck to you.