To tell if somebody is unsure about about some thing, they would use word spacers (erm), to slow down what they are saying and to give them time to reply or think about it. Word spacers can also be seen with somebody who is lying, when they haven’t fully prepared what they are going to say.
An unsure person would naturally pull a confused face involving facial tension which could include squinting their eyes or tightening their lips together. They may also repeat themselves quite often, in order to try and convince themselves that what they are saying is true or correct.
With terms of body language an unsure person may often find themselves ‘fiddling’ with a small object like a pen which would take their mind off what they are thinking about or talking about.