First, BE VERY CAREFUL WHEN MEASURING SO YOU DON'T ELECTROCUTE YOURSELF
If your not experienced or comfortable working with a meter or around high voltage get a pro to do this.
You didn't say what is happening with the heater. Is it not heating up at all?
If your not getting any hot water and you have 240 v at the source, then the likely cause is either an open top element or a defective top thermostat.
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Here's what I would do to check out a heater after confirming I have 240 v at the top theremostat.
1- disconnect the heater completely from electric by throwing the 240 v breaker.
2- take resistance measurements across both elements to make sure they are not open. If the top element is open you will never get any hot water.
3- if you read good resistance across the top element then turn power back on and CAREFULLY measure the voltage across the top element. You should get about 240 volts. If you don't get 240 v across it then the top thermostat is suspect.
Note: don't be fooled. One leg of each heating element will always read 120 v to ground. The other leg is switched to opposite 120 v phase when active.