The potential use of a free electron laser (FEL) amplifier as a diagnostic tool for relativistic electron beams is investigated. It is in fact shown that a simple relation exists between electron bunch length, energy spread, and the gain of a FEL resonant amplification. It is also discussed how the beam qualities can be derived from an actual experimental configuration. Finally, the possibility of combining the present methods to other techniques, like nondestructive diagnosis with short undulators, is also analyzed. ©1997 American Institute of Physics.