He picks it up, and looks at it from the back. Very often by looking at the back you can tell good jewelry from bad. You'll be able to see immediately if the piece is going to last you a lifetime, or not. The industry is very good in faking the shiny part in the front. In the back, you can see the thickness of jewelry, and also the care that was taken in finishing the piece.
Common sense is important.
Don't let yourself get blinded by the strong halogen light that brings the last hidden sparkle to life. Good jewelry looks beautiful in any light. Be aware, and be critical.