Attosecond pulses in the extreme ultraviolet range generated by the process of high-order harmonic generation (HHG) allows one to investigate with unprecedented resolution the unfolding of electronic processes in atoms and molecules. Recently the generation of train and isolated attosecond pulses has been demonstrated at free-electron lasers (FELs) operating in the XUV and soft X-ray spectral range. FELs offers remarkable peculiarities, including tunability, high-energy per pulse and multi-color operation. In this presentation, I will discuss the main challenges connected to attosecond metrology at FELs, showing, in particular, how covariance and correlation approaches can be implemented to completely reconstruct the temporal profile of the attosecond pulses generated at the seeded FEL FERMI. I will show application of this novel approach for retrieving the relative phase synchronization between the XUV pulse train and the infrared field with a resolution below 100 as.