Where the power wires first enter the water heater there are connections, usually wire nuts, where you can measure the voltage. First measurement, set your multimeter to 250 or more volts AC, then stick one probe inside each of the two wire nuts attached to the respective incoming wires. You should get about 230 volts here.
Next measure across the two terminals of each element. One should give a 230 volt reading. (Only one element is on at any given time, or both would be off when the tank is fully heated.)
Now turn the power off at the breaker panel. Set your meter to ohms. Measure across the two terminals of each element. You should get a value less than 100 (ohms).
A water heater must never be turned on until the inside is full as evidenced by water gushing from an open hot faucet upstairs.
Do you get coverage you can count on? From the bag of fertilizer? From your lawn sprinklers?