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.