Settings -> Apps -> Downloaded. Here you'll see a list of all the apps that you have downloaded from the internet.
If you open any one and scroll down to the bottom, you'll see all the permissions that the app has. An app that sends messages on its own would have the
send SMS messages permission enabled, which looks like this.
If you see this in an app, you have found the potential culprit.
Alternatively, you can download a third-party application like App Permission Watcher
, to watch which app has the
send SMS messages
Unfortunately, if you are not rooted, the only way you can stop the app from sending SMS messages is by uninstalling it.
However, if you are rooted, then you can edit the permissions of each app using your superuser controller.