  We present a setup for complete characterization of femtosecond pulses
generated by seeded free-electron lasers (FEL's) in the extreme-ultraviolet
spectral region. Two delayed and spectrally shifted replicas are produced and
used for spectral phase interferometry for direct electric field reconstruction
(SPIDER). We show that it can be achieved by a simple arrangement of the seed
laser. Temporal shape and phase obtained in FEL simulations are well retrieved
by the SPIDER reconstruction, allowing to foresee the implementation of this
diagnostic on existing and future sources. This will be a significant step
towards an experimental investigation and control of FEL spectral phase.
