Is there any way to tell if someone is reading my iMessages? I think someone may be reading my iMessages on another device?  Is this possible?  Is there any way to find out?
This is possible if somebody knows your AppleID password and uses it to log in onto another device. Changing the password and reviewing the list of alternative phone numbers/email addresses associated with your AppleID should be enough to resolve that (assuming that your new password is significantly better than the old).