If it were me I would test it in a bucket and crank it up for a day or so. Have never had a bad heater or one fail so I can't say for certain, but I hear stories of them exploding. If you have doubts I wouldn't test it with fish.
Plug it in and put your finger in the water lol. I’ve never had fish or me get shocked by heaters, and ive been working at a fish store for 3 years. We’ve had heaters blow and never hurt the fish. Now I have been electrocuted a few times, but not from heaters lol.
It all really depends on what components are in the heater, a lot of them are made safe now so if they were to explode, the contents won’t harm the fish. Fish also generally don’t get electrocuted because there’s no ground.
Regardless, I would just see if you could exchange it for a new one. Better safe than sorry.