This Old House Segment - Checking for level around a room

My son in law tried to stump me with an instrument that he spotted on a recent segment of TOH
Tom Silva was using it to show how you could go around a room and get a sense of how much a floor or ceiling was out of level. The concept is to measure up from the sill plate/floor or down from the ceiling/joists and mark a set distance (say 48 inches down or up) as you proceed around the room. Tom then used a digital variant of a water level starting at one mark as a reference point - to see how the others might vary. I believe that this might have been the tool used:

Alternatively he could have used other methods too like:
Using a rotating laser, made level marks around the perimeter of the room and then used his tape to measure up and down from the level line
Using a conventional level to mark the wall or studs for a level line and then measuring
Using a more conventional water level