You say it was unplugged? Don't really know if it's any good or not, if it were me I'd just plug the thing in, stand back and turn on the power.
If it blows up, then it's no good.

Just kidding.
Seriously, you could do a couple of checks with an ohm meter. Check from both power pins to the outside case to see if there's any continuity, if so, then throw it away.
Then check between the two power pins to ensure they're not reading a direct short. If so, then throw it away.
Also check to be sure that there is continuity between the ground pin on the plug and the case. If those three items check out satisfactorily, then it's reasonably safe to assume that it's ok to plug it in. But it's still not a good idea to hold the thing in your hands when you do it.

I never like touching anything metal under a spa unless I see, and know for sure that the entire thing is double grounded... (Grounded thru the power cords, and to the outside of the case with the separate ground wire).
Hope this helps.