Are you sure that someone is? If so, the most likely ways are that they got access to your Google account password or they installed tracking software. I would do a factory reset on the phone and change my Google password immediately. Make sure to use 2-factor authentication for Google, it makes it much harder for someone to gain access to your account.
The only way I can think of that you might be able to tell who is doing it is to go into your Google activity, where you can see the last 10 IP addresses that logged into your account. That only covers a small amount of time, but you might be able to identify a rouge IP address and go from there.