Right, I think the first method is much easier. Make the assumption the design was done correctly initially using established required loads by Code, and add them up on a PSF basis. That will tell you the capacity, at least for vertical loads.
If you're trying to find out if you can add more...
The roof was designed for a definitive load, most likely a uniform load like you say (PSF). ASCE 7 and Model Building Codes like IBC, and UBC etc., contain the loadings that buildings including their roofs must support. Your roof was designed to support its own self weight, maybe snow, a roof...
I'm surprised nobody has mentioned this, but what about striking out on your own with that PE? Take that license, get incorporated and try to land some work of your own.
