How can I find out if someone is spying on my phone

Erase your phone and set it up again with a different password.

Restore your iPhone, iPad, or iPod to factory settings - "A factory restore erases the information and settings on your iPhone, iPad, or iPod and installs the latest version of iOS or iPod software." - http://support.apple.com/HT201252

Also change your Apple ID password.

Change your Apple ID password - You may also wish to turn off "Find My" before changing information and turn it on again afterward. - https://support.apple.com/HT201355

Did you put some on there? If no then there isn't any unless you leave your phone sitting in public places with no passcode.