by guess or by gosh

anyone's guess

A mystery; something unknown or unpredictable.

guess what

Used to introduce a surprising outcome or one that the hearer is not expected to try to guess.

hazard a guess

To make a guess; to guess.

oh my gosh

oh my God!

you'll never guess

I have something very surprising to say; you won't be able to guess what I'm about to say.

your guess is as good as mine

I don't know; I have no idea.