Trust is built with integrity. You cannot have one without the other. When you have lived your life in such a way - with truth, honesty, courage and love you have the stepping stones toward trust.
Having said that, it is important to be aware of other peoples history and actions. If a person cannot look you in the eye, that is a red flag - be careful. Anyone who utters the words "trust me" is another red flag. Be aware of those that gossip, especially behind someones back.
Then there are the safety issues for which good common sense comes in to play.
If, as a child, we are neglected, lied to or abused in any way, that child grows up unable to trust.
So, how do we decide who to trust? I think the answer exists only with the individual. Listen to your "gut" feeling.
I hope this helps.
Blessings to you!