If you are allowed to do it then your suggestion will probably be the best way.
I think your concern is justified. There is chance that the more modern wiring has been installed in the more accessible areas and then just tied up to old wiring in the inaccessible ones. Definitely worth a check if you or a qualified electrician are allowed to do so.
I wonder if it might also be worth checking for ground on the receptacles? If they are all two-prong then this might be an indication that there is old wire servicing them. An open ground on a three-prong receptacle would also raise my suspicions. Similarly if you see three-prong GFCI receptacles everywhere (not just around water areas) then this might be another give-away.
What do the pros say?