Must as prediction? obligation?

Question

I was wondering, how you can determine whether "must" is used for prediction or obligation? For example, If I say "I saw the cashier asked an old woman for her id, so I figured he must ask for mine too", Does it mean the cashier has to ask me for my id(obligation) or the cashier is thought to ask me for my id??

Thank you:)

Answer 1

Your example doesn't seem idiomatic to me.
For the prediction, you might say
... so I figured he would ask for mine too.

I can't see any way to impose or express obligation on the cashier in your example. You are observing and making an inference - predicting.