Truth spells are really hard to do because most of the time we don't really want to know the truth. We want to know our version of the truth.
A combination of herbs (such as mullein and moonwort) and gemstones (like lapis lazuli, amber, and bloodstone) placed in the room where you're going to talk to her might be useful.
But I don't think a truth spell is going to work for you. I suspect that this topic has come up a number of times, and at this point it is a defensive discussion between the two of you. Discussions that turn defensive don't really allow for any kind of honest communication to happen. Good luck, honey.
If you trust your wife, believe her. If you don't trust your wife, couple's therapy may be the way to go.