I would always suggest using Mr. & Mrs. until they offer their first names. This shows them that you respect them and others, and that will score points with them.
Perhaps, in a situation before you are about to meet your SO's parents for the first time, you can ask your SO not introduce them as "my Mom and my Dad", but with the names they prefer to be called. Either that or just ask your SO what they prefer to be called, and start with that.
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