My landlord stopped making his mortgage payments in December and we don't know if he is in foreclosure. How do we find out?

Check with the county. A foreclosure process has to be filed with the county clerk.

If you have a follow-up question, please reply to this thread, don't start a new question. Once the house has been foreclosed on you will recevie a vacate notice from the new owner. That notice can be anywhere from a few days to a few months. Even if its only a few days, you can fight for more time.

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rachelpruet (https://www.askmehelpdesk.com/members/rachelpruet.html) disagrees: I called my county clerk & they said they had no way for them to find out. This is in Gila County, Az.

First, may I call your attention to the guidelines for using the comments feature found here:

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Just because your county clerk claims they can't doesn't mean my advice was incorrect.
Frankly, I think you were lied to. Real Estate transactions are generally registered with the county clerk. They should be notified in case of a foreclosure.