Questions questions questions This doesn't need to be an interrogation, think of a conversation where you can come to some agreement with the patron about what she is looking for.
Don't be Amelia Earhart. You do not have to fly the desk alone. If you are stumped ask for help or refer the patron to someone else.
At the very end, ASK if the patron is satisfied:
Help Mark find a professor
Help Betty with a history assignment
The Reference Interview by William C. Robinson
OLC Module: Reference Interview by Ohio Library Council
The Consumer Health Reference Interview and Ethical Issues by National Network of Libraries of Medicine