How to build a lie detector.?

Question

I work at a bottle store and want to test my workers if they are lieing to me or not. but the machine must not be very expencive..

Answer 1

Build anything that looks like a lie detector and tell them it's a lie detector, this will make them tell the truth.

Answer 2

Look at the tea leaves in their cups .....  it's no less accurate than a polygraph.