Tail him if you can, it'd be best to have a camera on you to grab evidence as well. Also it may be within your interests to find out this womans scent, when she comes by walk out and start conversation with her, if possible, use it as a excuse to take note of things about her that you could later identify on your father. It may also be good to find out what /she/ thinks of him, or at least to the extent that she'll let you know, any abnormalities should be kept in mind. If your father has any certain patterns, take note of any changes.
tl;dr sniff the bitch and then sniff your dad, if they smell the same he's cheating. Ace detective.