Your Timeline tells you where and when to focus your attention in order to verify your partners story of where your partner is going.
You can ask a friend to follow your partner, that's not known to your partner. (Your partner doesn't know them or their car.)
You could also rent a car for those times when you need to verify your partners where abouts and keep it parked close by.
The other option would be to hire a Private Investigator to surveil your partner at the suspected time, from your Timeline/Log.