Home Inspectors follow the standards of practice of either ASHI (American Society of Home Inspectors), InterNACHI (International Association of Certified Home Inspectors) or CREIA (California Real Estate Inspectors Association).
All three standards call for an inspector to check a “representative number” of electrical outlets. But what exactly is a “representative number”? It is rather vague. I have heard some inspectors say it is one outlet per room.
I say ALL outlets that are available should be checked. If an outlet can be seen, and is not obscured by furniture or belongings, and if it does not have something plugged into it, then it should be checked, because you never know.
A case in point is this set of 5 outlets all next to each other. This was found in a living room in the San Fernando Valley.
Four of the five outlets tested to be good. The one outlet in the middle showed an open ground. If I had just tested some of the outlets, I would have missed the single outlet with a problem.
Choose an Inspector who tests all available outlets, and not just a “representative number”.