To the OP,
There are very good, and insightful posts here. But, let's look at an example.
At the farm there is a very well trained blind horse, whose owner can do anything with. She's putting her feet up on his neck, kneeling in the saddle, etc., etc. This horse takes it all and doesn't flinch.
Yesterday, she put a newbie on his back. O.k., this horse is blind, he did not see her approach, he was not able to read her body language. But, he KNEW immediately that this person did not know what she was doing. How?
His owner is a very experienced rider and is very relaxed and loose in the saddle. This rider was very tense, nervous, tight in her whole body. That was his first and immediate clue. That tension was evident in everything she did with her hands and her legs.
How did this horse react? He became tense as well. His head was high, his pace was quick, his ears were focused directly behind him (not pinned). Thankfully, by nature, this horse is laid back.
Now, when I put a green rider on the back of a horse for the first time, the first thing I have them do is take a few deep breaths and relax. This is before we take one step. I usually use a horse I know, like my Honey, because I know that she will stand perfectly quiet until she is given the cue to walk on. This is what I did with this same rider immediately after she got off the other horse. She needed to learn a proper dismount and I knew that Honey is patient enough for this lesson.
Oh, and one more caution I should put out there. NEVER, ABSOLUTELY NEVER get on a horse you do not know without another EXPERIENCED person there. We recently had someone end up in the hospital because they did not follow this protocol.