I keep it really simple. Tape measure while on the bike. 1/16" or greater - it comes off and bagged and marked at its elongation, When new chains stop working it is time to decide whether to use those older chains and run all my cogs and rings into the ground or swap out the cassette and chain and prolong the life of the chainrings. (At 14,000 miles for this bike with its three cassettes and two wheels, new chains are still running just fine. )
Take the chain off and clean it just to measure it? I know any worthy bike owner regularly takes his chain off and cleans it. I don't. Lube and wipe. Ride. Seems to work. (See above.)