Examine the design of the chair. The Bertoia chair is made of steel rods. It has a solid appearance, yet a contrasting airiness. If the chair you are looking at seems flimsy or heavy, then it is probably a fake.
Determine the manufacturer of the chair. Knoll is the manufacturer of the genuine Bertoia chair. If the manufacturer is not Knoll, it is not a Bertoia chair.
Find out where the chair was made. Bertoia chairs are made in Italy. A fake Bertoia chair may not be manufactured in Italy.