For projects where we do not receive all construction layers, I usually try get information about the different layer materials and their thicknesses from detailed wall/roof/floor sections. I then get the respective material U-values/R-values from reference tables. This question has some good resources that can be used for reference.
If such drawings are not available, I usually request this from the project team. You may want to document the layers details from the reference project drawings in the energy modeling report that is submitted to GBCI if you want to be extra safe about review comments (review comments about constructions are quite common).
Table G3.1 (section 5) describes the requirements for modeling the envelope properties for proposed design and baseline models.