The whole idea of trust is stupid.
You watch what people do and act accordingly.
If you have known them for a while maybe you can leave $5 in your drink holder or pay for their lunch and see if they offer to get yours later. Little shyt.
Once they pass the test with the little shyt maybe you can leave them in your house while you make a quick run or loan them an article of clothing.
What I'm saying is: you have to see them do the right thing by you on a consistent basis before you can determine how far you can trust them.