We had a electrician install a 250-125 volt, 30 amp socket,outlet4 prong male in our wellhouse,back in ,1999,this was never attachedto the wellhouse wall, there was no cover on it, this was just sitting, on the dirt floor,there is slight surface rust, on the back of the metal outlet, several years ago I wrapped,this up in heavy plastic, and put a brick under it, is this oulet safe? is there a way,to test, the outlet itself? we do know that, no moisture got into it,but concerned that dust or tiny bugs,may have compromised it? I will also call the electrician company that installed it,to ask there opinion. Since this is for a lot of electrical juice,I'm concerned. Any thoughts? thank you, for your imput. The face of the outlet,is normal,no rust or corrosion,just surface rust,on the back plate.

So the electrician that installed it violated two codes, did not attach to the structure, and left it with no cover? If you see surface rust, then there is corrosion inside the receptacle, and needs to be replaced. I would suggest you call in an electrician, not the same one, since seems they do not know how to install an outlet properly, mount the box onto a structure at a level that is higher that water level , replace the receptacle device, and install a proper cover.

Please direct the new contractor to take out a construction permit and
have his work inspected. If that had been done twenty years ago you might
not be doing it fixing it now...