Water Heaters - Rheemglas Fury electric....checking Tstats?

Hi, I have a one year old 50 gallon electric rheemglas fury water heater. We are running out of water sooner than usual. Both elements have been replaced with the better grade stainless steel elements within the past 2 weeks. After filling tank, flushing air and turning power back on, I seem to only have 240 at the top thermostat. Guy Gray (our local Rheem dealer) tells me that only one element heats at a time. They say, water in the bottom of tank would heat initially...as water exits tank, if not to temperature, the top kicks in. After a new install of the bottom element, the top Tstat is the only one that shows any power. How do I determine if bottom or top tstat is bad since they possibly alternate current between the two at different times?
Thank you

If the entire tank is cold, then the upper element will fire. As the upper portion of the tank reaches the setting of the upper stat, power is then diverted to the lower stat and element. You can check this with a volt meter.

Generally thermostats fail long before the elements. So it would be a good idea to just replace the stats whenever elements are replaced. In fact, this is standard procedure for most plumbers. Besides, stats are cheap and easy to replace.

How do I determine if bottom or top tstat is bad since they possibly alternate current between the two at different times?

Turn the power off pull the wires to the elements off and Ohm out each element with a meter

Thanks, everything working fine now....nice site also