I wouldn't just assume that it is correct in the first place. If this guy is being asked to evaluate its load carrying capacity, and he has no experience with this, what are the chances that whomever did the original design was another inexperienced engineer for this application? Check the whole thing for its actual capacity instead of referring to minimum loads in building codes and working backwards to a uniform load and then uniform pressure.
I would check all the failure modes, e.g. shear, bending, delfection, axial, interaction of axial and bending, headers, jambs, connections, etc. and the load capacity will reveal itself in your results. Try to base that on the in situ conditions as much as possible also since you don't know that the builder followed the plans, if you even have plans to look at.
I think you should hire someone, pay him or her the couple hundred dollars to write you a signed and sealed letter telling you what you need to know and go home and sleep well at night.