Bogus parts often come with full documentation, including FAA form 8130. Sometimes the physical appearance of the part will give away it's true origin. Lack of manufacturer's markings or unusual weight, texture or color (due to differences in materials and manufacturing processes) are often the only clue to the part's illegality. Besides the mechanic, the receiving inspector and parts room personnel must also share the responsibility for identifying any suspected unapproved parts.
Thou shalt mind thine altitude,lest the ground reach up and smite thee.