There are a multitude of things to look for and do when inspecting a fire sprinkler system. They range from checking design aspects such as piping size and the number and location of sprinkler heads to inspection and/or operation of of the water flow switch, flow alarm, check valve, pressure gauge, test/drain assembly, etc. This is covered in NFPA 25: Standard for the Inspection, Testing, and Maintenance of Water-Based Fire Protection Systems. Section 4.1.2.3 states that "These tasks shall be performed by personnel who have developed competence through training and experience". Unless you have that training and experience (which you may have, I don't know), I think you're doing the buyer a disservice, and taking on unnecessary liability by doing a partial inspection. It seems to be the same as a commercial system, but in NY you have to be a licensed plumber with sprinkler system certification (NYC at least)to perform the REQUIRED inspection. This brings a whole new set of questions to mind... number of heads:size of main. I can't see how an average main can supply adequate water necessary.