I noticed on my phone bill that I have a lot of sent SMS messages which I'm not aware of and can't find in the sms app. Is there a way to find out which app is sending these messages? And how to view the details of these messages on my android phone (I'm using Galaxy S7 Edge).
Well first take a look at what is your standard SMS App. If it is anything that is not native to your phone (e.d. third party App) or something you installed away from google play store (e.d. untrusted source) it can be some sort of malware.
But before jumping to conclusions it is recommended you check every app in your phone and try to see which apps are even capable of sending SMS. To check if the app is capable of sending SMS, go to your Settings -> Apps and than click on each App individually. It should display a list of permissions the App has on your phone. If you see that the app has the permission of sending and recieving SMS, but is not the App used normally for communication, uninstall it immediately. Of course, this doesn't include Apps such as Viber, because they can indeed send SMS messages.
Hope this helps.