In case you are suspicions about your partner, you just need to check their phone. This is very easy to do. For example, you can read the text messages they send and receive. Luckily, there were a lot of spying apps invented. And spy phone app
https://www.hoverwatch.com/free-cell-phone-spy would be useful in your case. This application works invisibly to the user, but with an advantage for you. Apps that can do this are increasingly popular and are used by becoming millions of people worldwide