This helps me - your mileage may vary:

I either have DID or I am making the whole thing up.

If I have DID, then I have suffered severe trauma.
If I don't have DID and I am making the whole thing up then I have almost certainly suffered severe trauma because really, I can't imagine a person making up alternate personalities who has not undergone severe trauma. I mean, if that seems like a good idea...

Since DID is a result of severe trauma...well, you do the math.