Nor do I, but that was a long time ago. I don’t even remember disclosing my SSN when we got our divorce some eight years later.
For what it’s worth, the actor Ken Wahl, who’s married to the model Shane Barbi, is (according to statements by everyone involved), also sleeping with her identical twin Sia regularly.
Maybe Shane and Sia are sharing Ken without his knowledge.
Haul them into court where they get threatened with contempt charges unless they identify themselves as to which is which?
What exactly constitutes this? Is being engaged for a long time sufficient? Or do you have to be telling people you are actually married, calling each other husband and wife? Or is it more legalistic, like buying a house together, etc.? Just how concrete does your “holding yourself out” as married have to be to consider it a common law marriage?
For instance, if you were living together, had a kid, co-owned a car or a house, were engaged and wearing rings, would that be enough? Or do you have to actually take the step of telling everyone you’re married?
The people who know you have to believe you are married, most commonly because you have told them you are, so it’s usually your first point there. I suppose it’s not essential, but the most common evidence would be that you refer to each other as “husband” and “wife,” or “spouse” in the third person, or that you introduce each other as “my husband/wife/spouse.” If such evidence is not available, the courts will consider some of the less direct evidence that you have mentioned.
I presume all possible puns resulting from the juxtaposition of Mr. Wahl’s first name with his wife’s surname have already been made? 
We did have to give ours when we married in FL not too long ago. I am sure that this is one of those things that vary a lot from state to state.