I have one even better, just to show how you can't be sure... Replacing an outlet. Shut off the 2-pole breaker for the shared-neutral circuit it was on. Outlet

I have one even better, just to show how you can't be sure... Replacing an outlet. Shut off the 2-pole breaker for the shared-neutral circuit it was on. Outlet tester showed the outlet was off. Pull the receptacle out. Used a multimeter to test each wire contact to ground; all were 0v. Only when I disconnected all the wires did one of them develop potential because somebody had apparently tied a neutral wire to a hot in another box, but I still don't understand the situation and had an electrician address it. So even testing each individual wire isn't certain unless they're all hanging free, unless you make the assumption that things were wired correctly in the first place.