I agree with dancing arabians about how a horse expresses trust. I took my nervous nelly boy out on a solo trail ride. I somehow decided that it was a good idea to ride him over a bunch of rotten slash. So, he was sinking through this rotten wood as I tried to navigate him out of it while trying to avoid what I thought was really bad section of it. I could feel him tense and wanting to rush willy nilly back to the barn but he listened to me and my cues. We got out of the slash and went on a little trip farther away from the barn. That was when I realized that he trusted me a little bit.
To me, you don't know a horse trusts you until the horse is in a situation in which it has to trust you. A challenge is presented and the horse can either leave and say screw you or it can say I am not comfortable but I am doing this with you. Walking around the barn does not in my opinion meet this criteria. Trusting me to feed you is not a big deal ( I do that everyday) trusting and listening in a new situation where the horse is not comfortable means more.