It is increasingly difficult to answer this question. However, trusting anyone is a risk you will have to take. If you want to form strong and lasting relationships, you have to trust everyone.
No two persons are the same and everyone carries around ideas of what is best for them. Therefore, we cannot think that at all times we can rely on everyone.
I was briefly apart of a discussion about two recent incidents recently, one a murder suicide and the other a murder of a woman. In both instances the men were the perpetrators. In the discussion the question of whether or not you can really know someone came up. Can you really vouch for everyone you trust and do you really know everything about them?
At the end of the day we each have to decide who to trust: just make sure that you are trusting the right person with the right thing and for the right reasons.
I think the greater question too is, what can you trust people with. There are many different things that we value, can we trust everyone with them?
While we cannot trust everyone with everything, we can choose who we trust with what. You need to know enough about someone to determine what you can trust them with. If someone threatens you or abuses you, can you trust with your life? If someone is careless with their possessions, can you trust them with your own? If someone never or hardly ever make time for you until they want something, can you trust them with your friendship? If you try to speak everything into being but never act, can you trust yourself with your dreams?