I(36M) left a stack of condoms under the bedroom, how can I find out if my wife(34F) has been cheating?

Hello, I'm usually an optimist. I've been married to my wife for 10 years and generally we get along well. We have three kids and when I put them to bed last night I wanted to check that there was a stack of condoms I left under the bed. They were gone. I searched everywhere and asked her about them and she said she'd looked but had no idea where they were. I know that our sex life isn't as good as it was so I'm starting to suspect infidelity. I know of no other evidence of this. How can I find out?