How can I tell if someone else is on my iPhone

Hello ~ Take a look here by clicking on below and working through the information there…

See who has access to your iPhone or iPad - Apple Support

Post back if you need further assistance.

~Katana-San~

Well, it all depends on exactly what you’ve shared.

If you are the only person ever to have used your Apple ID, the iPhone is in your possession, never been jailbroken, and never used by anyone else, then nobody else is “on” your iPhone.