I want to save you a lot of stress. There are indeed tools that you can install on your significant others (boyfriend, girlfriend, husband, wife, side chic) phones to monitor them, but the real question is, what will you do when the thing that you fear happens?

My advise for you is this, if you know you don't trust your significant other and you are not willing to do so, don't add to the natural stress in the world by becoming a monitoring spirit, just walk away. Heaven will not fall if you peacefully separate and find someone you trust. If however you feel you cannot trust anyone and will always be suspicious of them, my friend, relationship no be by force.