The CDC doesn't recommend it in most cases but you can get one. I'm glad you asked because I didn't realize MMR boosters were even available for adults. My mom has cancer and is on chemo so I think I should get this. If you don't fit any of the cases that the adult booster is recommended for I suppose you could just say you don't remember getting it as a child or that you have a close relative with cancer and see if they sticks. But, I mean, I'm not sure if you actually need to justify it. Maybe you can just walk in and get it.

I am currently ttc and they made me get tested to see how my antibodies were for measles, mumps and rubella (and some other diseases we were vaccinated against in childhood). My rubella antibodies were pretty much non existent now. It can wear off over the decades. Was told I should get the MMR booster before becoming pregnant. So I guess maybe its not a bad idea for us adults to get our antibodies tested, or maybe go ahead and get the booster if its available in your country or area if you are in your thirties or older. Especially since a bunch of parents are no longer vaccinating their children, the herd immunity isn't protecting those who either were unable to get vaccinated or the antibodies from infection/vaccination have worn off. My stepfather completely lost his hearing in his right ear because he got measles as a boy (before the vaccine was created). So even ignoring the horrible painful rash you get, it can also make you deaf, blind, or kill you. A lot of us have forgotten how damaging these diseases can be, we've been spoiled by our vaccine protection. Even if you got your MMR shots as an infant it does wear off and you will no longer be protected by herd immunity.

edit: i wrote father in law instead of stepfather

Seriously, I'd like to know too. I've always thought that you got them and then you were done. But maybe that was counting on there not being a bunch of disease vectors walking around.

Yes just get your titers tested, that will let you know. It's a simple test.