There is no way to initially tell which numbers are landlines if they are imported via CSV or ChMS integration. However, after you send the first broadcast, if it doesn't go through (for landlines or bad number), Gloo automatically marks that person as Unsubscribed.
It does use credits for the initial message to a landline or bad phone number since there's no way to determine ahead of time.
The good news is that what we've seen so far is that it's a minimal credit expense, so it shouldn't have much overall impact, and it won't happen multiple times with the automatic "Unsubscribe" afterwards.