Chimera cannot figure out the resolution of a density map, and the density map file formats (CCP4, MRC, BRIX,….) do not specify the resolution in the file.  You need to get that information from the researchers who calculated the map.  For electron microscopy the resolution is usually calculated by computing two density maps each using half the experimental data and comparing the two maps.  For X-ray crystallography the resolution is based on which diffraction spots were observed.

If you want to find out the density map grid plane spacing use the Chimera Volume Viewer dialog menu entry Features / Coordinates and look at the Voxel Size value.  This is not the resolution.  Usually it is approximately 1/3 of the resolution.

Tom