How to check phone for spyware

While there is a possibility for someone to have compromised your iCloud ID, the ability to install spyware would require someone to have unlimited access to your device for an extended period of time, as well as your Apple ID and password.

See this to change your password for your Apple ID and make sure that you activate 2 Factor Authentication.

If you think your Apple ID has been compromised - Apple Support The information on how to change your password and use 2 Factor Authentication are both located in this support document.

If someone is able to log into your iCloud from another phone that will have nothing to do with spyware on your iPhone it means that someone has found out your Apple ID password (was it complex) and you are not using 2-factor authorization. So change your password and turn on 2-factor authorization.