You can determine the materials applied to a given element by looking at the Type Parameters (use Tab Select to select elements in linked models).  See below for the process to access the material parameter for Walls (Materials for other kinds of elements can be accessed similarly).

Once you know what material is applied, you can adjust the reflectance of that Material in Material Mapping. Note if you’re working with a linked model, be sure to select the appropriate model at the top of the dialog (see below).