What like anything?
People can be trusted in different ways. One person may be good to trust with keeping to their word with plans, another good for keeping secrets/info private, another may be upfront & honest with you, etc. That being said, I usually trust consistency. I'm willing to blindly give trust to people I only know somewhat well, as long as it isn't with anything too serious. Then I lose that trust over time if I see there's good reason to.
Shh, I'm testing something.
Ę̵̚x̸͎̾i̴͚̽s̵̻͐t̷͐ͅe̷̯͠n̴̤̚t̵̻̅i̵͉̿a̴̮͊l̵͍̂ ̴̹̕D̵̤̀e̸͓͂t̵̢͂e̴͕̓c̸̗̄t̴̗̿ï̶̪v̷̲̍é̵͔