How to find out

Married for about 10 years now. Often talked about cuckolding with the wife but she never really responded to it. In the beginning we had sex several times a week. Now for the last 2 years about nothing anymore. Maybe once a month and only when I start. Also in the beginning her pussy was tight for me but now when I have the occasion for sex her pussy is really loose. If I start about cuckolding she gets angry and replies me that it is only a way for me to find another wife. I assured her already many times that it isnt. Furthermore she likes to go out alot lately. It started with once a week but then it was about 3-4 nights a week. Nowadays its been back to 1 or 2 times a week mostly in the weekends. However when she goes out now she returns at 8-9 in the morning whereas before 2-3 in the night.
What might be a good way to find out if she is fucking other men.

wait, she comes home the next morning and your wondering if your a cuckold? maybe, she is not wanting to talk about it b/c she feeling bad about what she is doing. When she goes out what is she wearing ? normal everyday clothes or all "dolled-up" clothes? do you notice more perfume/more make-up?

I think you already know the answer to your question, best way to find out is really "follow" her it may sound creepy but your the husband ! find out and then decide how "you" want to handle it.

Suck her pussy when she gets home!

Install an app on her phone to track her GPS location.

My ex started going out, 1 night a week and then turned it up to many nights a week, clubs, including strip clubs for ladies and for lesbians and she too didn't want to discuss anything, as she got off keeping me in the dark. I later found out, via some love letters she was throwing away, in the kitchen trash I had to take out, she was having many torrid affairs with both men, but mainly with a great many women, including office coworkers.
She is now out the closet and lives with a woman she loves.