1 Chronicles 12:17
"And David went out to meet them, and answered and said unto them, If ye be come peaceably unto me to help me, mine heart shall be knit unto you: but if ye be come to betray me to mine enemies, seeing there is no wrong in mine hands, the God of our fathers look thereon, and rebuke it."
Knowing whether or not to trust someone is certainly a hard thing to figure out.
It might be that you have met someone for the first time and you're not sure about them. It could be that you moved to a new area and are not sure whom to trust. It may be that you are attending a new church and are uncertain about the people who attend. It could also be that you just hired someone and are wondering what they will become. The landscape of meeting new people and trusting them may be different, but the uncertainty of trusting them is always the same.
David faced uncertainty about people when he first took leadership in Judah. Saul was dead, and David didn't know whom he could trust. There was a strong following of people who wanted Saul's family to keep the throne, and David didn't know whom he could trust. The men of Benjamin and Judah came to David for they wanted to join up with him in making him king over all Israel. David wasn't sure if he could trust these men, but how he dealt with them is the same way you should deal with people when you are uncertain about them.
First, do not prejudge a person whom you've just met. It is not fair to have your mind made up about someone who you don't even know. You could be making a wrong judgment about someone who is truly a good person. Give a person a chance to show who they truly are before you make a judgment about them.
Second, let God reveal a person to you. David told these men that God would look on their hearts and rebuke them if they were misleading him. David understood that he didn't know their hearts, but God did. He understood that God had the capability to protect him from those who would try to hurt him if he kept himself right. Likewise, you must trust God to protect you from those who would hurt you. If your heart is right towards God, then He will protect you from those who have ill will towards you.
Third, don't open the kingdom until they have proven themselves. David said that he would allow his heart to "be knit" unto them if they came peaceably. David wouldn't know whether they came peaceably until some time passed. He was letting time be the justification of whether to open the kingdom to these people. Just because someone may immediately seem trustworthy does not mean they should have the strings of your heart. Everyone should prove the worth of your trust. It is your heart that you must protect. Certainly, you should give them a chance to prove themselves, but you don't have to open the kingdom and let them have full access to your heart. Let time prove a person.
Finally, give others time to trust you. Notice that David said to them, "Seeing there is no wrong in mine hands." Not only did these men have to prove to David that they came peaceably, but David had to prove to them that they could trust him. Just because you feel you should be trusted doesn't make it right. Trust is a two-way street. They need time to trust you. Don't get frustrated with them because they don't immediately trust you.
Uncertainty with people is a tool of protection that God gives us to keep us from wrong people. Don't disregard that uncertain feeling in your gut. Listen to it, but don't let that uncertainty make the final judgment. Let time reveal how much trust you should give a person.