If you suspend your girlfriend or boyfriend in cheating, I think it will be normal to spy a little. I even know stories where the suspicion turned out to be true. By the way if you want to check your partner's smartphone, there are a lot of special software for this purpose. Just visit spy-apps-software.com/review/mspy site and research all info with examples and guides.
I do not accept such actions as spying but I know that there are such apps that are used to spy on your phone. Here is the list https://celltrackingapps.com/how-to-track-secret-conversations-on-facebook/. But sometimes it may be a good idea to use such apps, for example, if you want to see what your children do with their phones or track their location.
It’s very tempting to want to spy on your partner, especially if you suspect them of cheating. There are so many ways to monitor their behavior in this digital age, and adding an undetectable app to their phone takes a matter of a minute or two. Who could resist the idea of keeping an eye on their partner's Facebook activities, Whatsapp chats, browsing history, emails, texts, calls and location