Checking real availability of MS Online services is a little hard, especially for Teams for several reasons:
Teams is somehow a “hub”, makeing use of several other services you would have to monitor as well. I added some overviews that might help to separate the services you need to check.
Teams make use of some kind of “microservices” internally. So even when being able to log in you might have service degradation because messages can’t be sent, calls can not be made, presence service is not working, or you get a nasty error message when logging in, …