Without knowing which test equipment you have, it is difficult to give the steps you have done in the past. However, if you have a snap around ammeter and voltmeter, this is the routine I use. First check for 240 v. power at the input to the upper thermostat. Then check for power below the high limit switch. Then check for 240 v. power to the upper element. If it has power, then check for 240 v. between the lower left and lower right terminals on the thermostat, and then use the ammeter to see if it is using power, at least 17 amps. And if the test between the two terminals did not show 240 v. or if it showed 115 v., then the lower element is burned out. If it did not have power, then the tank should have some hot water so perform the same tests on the lower element.: although I have done this numerous times, I can never remember how to check the elements and therostat on an electric water heater.
