The eyes never lie
Believe it or not, if you want to find out if someone is lying to you, the best way is to just stare at them. We mean by just looking at them and remaining silent, they will usually keep talking and dig themselves deeper into a lie, eventually revealing the truth.
Now, this really only works on normal people, we are sure that trained assassins or CIA agents will be able to deal with this with little problem. Eye contact with silence is a deadly weapon though and can be used on the untrained liars out there to get the answers you seek.