It can be hit and miss with elements. If after replacing the breaker and it still trips you can unhook the power leads to the element and fire the spa up. If the spa trips it's not the heater, if it doesn't trip it probably is the heater. Then hook the heater back up and hit the power if it trips there is a 95% chance it's a bad heater.
Still could be a few other issues causing the breaker to trip but the heater is a good start.