Keystroke logger on computer, phone records. You can also use a cheap cell phone as a GPS tracker - just throw it in her car (ringer off) and you'll know where she really is. You can also use a voice activated tape recorder put under her seat to find out what she's talking about in her car.
Make sure you're really ready -- not for the faint of heart.
Goodluck - I'm always of the thought that I'd rather know the truth, no matter how painful, than make decisions based on lies.
