From Allan Tsang, 88owls.com:

One thing we all despise is having someone lie to us. Here are a few ways to help you determine when someone is lying to you - or at least not telling you the whole truth.
 
A common sign of partial truth-telling - or lying - is when people do not look you in the eyes. 

Another more reliable but less known clue is when you ask a question that requires a visual recollection.  If they look towards their right, chances are they are accessing their creative storage shed (therefore either lying or not telling you the complete truth).  However if they look to their left and upwards, they are accessing their visual memory bank and therefore trying to recollect facts.

These signs, however, cannot be taken out of context. Subtle changes need to be taken into account.  In other words, you have to do a calibration beforehand...

How do you calibrate?  You ask a few non-essential questions and watch for responses.  Look for motions or changes in position such as a finger over the top lip, a crossing of the arms, of the legs, a shift in posture, leaning back.  This is stuff poker players study...the "tell."
 
Try it when you are having your initial interview with a consultant. 

In fact, try it on your kids.

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Allan Tsang is the founder of 88owls.com, a community of business consultants with more than 10 years of industry experience. 88owls.com matches business consultants with businesses seeking consulting services.  You can follow 88owls on Twitter @88owls.