Like say if I ask someone what’s are you using that for and they say guess I’m looking for a good idiom that indicates I’m not interested in guessing like “I’m not a mind reader” springs to mind, but does anyone have any others similar to this ?
I have no idea. That's the most used idiom for this situation. It implicitly means you don't know and don't want to guess.
To not be aware of or know more information about something.
A: "Is Sally off today?" B: "Oh, I have no idea. Ask Elaine, she'll know."
Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.