@Alexypalexy,
It is pretty easy to track him or her.
You simply download any one of a number of trip apps on your cell phone (or a cheap cell phone you buy for this purpose). Then you start the app on the phone just before he starts his ride, and hide it somewhere on the bike that he won't find it. Then when he gets back you recover the phone and look at the gps track.
Of course, spying on your partner is unethical and probably illegal which raises the question...
If you don't trust your partner, why don't you just dump his sorry ass? If he finds out that you are spying on him, he will surely dump yours.