So, I am embarrassed to say that I've gone several years without referring my fiance's mom to any kind of name. When I first met her, she never told me what to call her and I didn't realize that until a year or two later. I feel like it'd be awkward to just ask! In Asian culture, many different ethnicity have certain names for an older person.
One time we happened to have a conversation about this topic about who called what who, and his aunt (she's 20 years younger than his mother, she's my age) said, "Wait a minute, what do you call his mom?" and his mom just happened to walk by and I said, "I don't know, she's never told me." and she just laughed as she passed by. I should've asked her then what I should've.
So now I'm thinking I should bring it up when it comes closer to the wedding LOL. I do know that his brother-in-law calls her "mom". Do you wait to be invited to call them "mom" and "dad"? or do you ask?