There are many gestures that indicate a person doesn’t know.  The verbal phrase might be “search me” but the body language gestures are unmistakable.

Arms and hands

The most common gestures involve the arms. As in the attached picture, the woman is holding her arms out with palms up. There are several variations of this gesture but they all involve palms up and the shoulders somewhat raised. It may manifest itself with a shrug.

The hands can be level as in this picture or they may be uneven due to one shoulder being much higher than the other.

Another common gesture with the arms and hands is to have one arm across the stomach causing a kind of shelf on which the opposite elbow is propped with a finger either on the chin, cheek, or even in the mouth with a biting expression. 

Facial expression

The usual facial expression to go along with the hand gestures is one of slight confusion. The mouth will be shut or sometimes it will be pulled to one side indicating the person is thinking. The eyes are normally wide open and the eyebrows will be high.

Alternatively, the eyes might be looking to the side as if the person is looking for some clue or playing a kind of guessing game with you.

General posture

In most cases when you see this gesture the person will be standing.  It is possible to show it while seated, but it is far less common.  

What to do

When you see this expression, you need to take the circumstances into account.  If the person was just asked which movie she wanted to see, the connotation would mean that she really does not care. On the other hand, if she was asked about a new technology, it may mean she really does not know. 

In either case, the best response is to get the person to talk. The gesture itself is clear, but the resolution needs to come through dialog. Avoid a mirroring gesture when you see these things. Instead, offer specific alternatives to help the person verbalize a preference.