  We discuss the well known Einstein and the Kubo Fluctuation Dissipation
Relations (FDRs) in the wider framework of a generalized FDR for systems with a
stationary probability distribution. A multi-variate linear Langevin model,
which includes dynamics with memory, is used as a treatable example to show how
the usual relations are recovered only in particular cases. This study brings
to the fore the ambiguities of a check of the FDR done without knowing the
significant degrees of freedom and their coupling. An analogous scenario
emerges in the dynamics of diluted shaken granular media. There, the
correlation between position and velocity of particles, due to spatial
inhomogeneities, induces violation of usual FDRs. The search for the
appropriate correlation function which could restore the FDR, can be more
insightful than a definition of ``non-equilibrium'' or ``effective
temperatures''.
