How do I tell someone something that I'm not even sure of myself. I don't want to sound idiotic?

If i was in your position i would make sure from the thing because you don't want to mess with the person mind if you are not sure and if are wrong and tell the person the wrong information the outcome could be nasty because of something you are not sure of. if the situation is very serious and you don't know to make sure of a situation you should sit with this person and tell them i am not 100% sure but i have something important to tell you .................. you tell them the situation and tell them why you think it is true then they will understand why you thought that "something" and your point of view on the situation.

if you gave me more info i could have help little more :)

Hope this helps:)