How can I see if someone has been in my phone taking my hidden photos?

You may want to check, if the Hidden album is still protected by a passcode. This is the default and your Hidden album should still be protected, unless someone, who knows your passcode, has changed this.

See: Delete or hide photos and videos on iPhone – Apple Support (UK)

"The Recently Deleted and Hidden albums are locked by default. You unlock these albums using your iPhone authentication method—Face ID, Touch ID, or your passcode.

To change the default setting from locked to unlocked, go to Settings  > Photos, then turn off Use Passcode."

Unless you handed phone unlocked to someone with the Hidden photos on the screen, no one can access your iPhone, much less your hidden photos.