How can you tell someone if there are really telling the truth?
Have they ever lied to you? If no, you shouldn't be THAT suspicious, and do an underground investigation.
By the look in they're face and by the tone of they're voice
Mostly by their actions and expressions. You can tell when someone's lying sometimes by looking at them. Some people sweat a lot, stutter, or avoid eye contact when they're lying.
they could be smiling if not telling the truth
Add a gun in their face to the equation. This might help.
If they let you touch there ear if is hot lying cold not
Look straight in the face