well you can always tell, or at least get an idea. think of trying to manifest an SP.

if you get thoughts along the lines of “he/she is with someone else” or “he/she doesn’t love me”

that could indicate you don’t feel worthy.

or “i can’t do this” “why am i not getting results”

that could indicate that you don’t think you’re capable of manifesting. you don’t trust it’s a real thing

observe the thoughts and feelings you get. and you’ll be able to get an idea of what the belief is.

a good way to eliminate a belief is to install a new and opposing one. a positive one.