If YES it is an iPhone:
- Go to iCloud.com
- Go to the Find My iPhone page and see if there are any devices you don't recognize.
- It's your choice to lock them with a password or remote wipe them...
(I'm not suggesting this, I'm just saying it's an option). - Then immediately change your password.
You can also see what other devices are connected to your Messages in the Settings app. But you can't do anything but disable them.