I've been asked in the past to do this also. I usually look inside the panel, and open about a dozen randomly selected outlet boxes. I also look at any larger appliances, such as electric dryer or A/C unit. I then write a report about only what I've seen, and whether or not it is code compliant. I'm also sure to use phrases like it "appears" to be mechanically secure, or I did not "observe" any discoloration or degradation of the insulation, etc. So I'm not telling them that the neutral is not overheating, just that I didn't see the signs in the places I looked. Then of course, the usual disclaimers as you've mentioned, about not being able to see everything and not being responsible for any issues not discovered or not yet known, etc. Also include some recommendations if you notice that the devices aren't pigtailed or aren't CO/ALR rated.
John
