The spatially resolved spectral (S2) imaging method is applied on an active microstructured fiber, with a multi-filament core (MFC). This type of fiber has been designed to be the last amplifying stage of a source for a long range coherent lidar. Studying the influence of the bending radius on the modal content with or without gain, we demonstrate that an upper-bound of the high-order modes content can be found by performing the S2 imaging on the bleached fiber. S2 imaging is then used to verify that the output beam of the MFC fiber can be made effectively single-mode. We also show that it can be simply adapted for measuring the fiber birefringence. Finally, a comparison of the MFC fiber mode area with that of a standard large mode area Erbium doped step index fiber illustrates the interest of the MFC structure for high power amplifiers.