Thank you 35hotrod.. I've been twistin' wire nuts since '87 myself.
If you have orange romex it is fairly recent work as the color code for romex hasn't been around for long. You will have two hots and a bare copper ground conductor in that romex. I will go out on a limb and say if a licensed electrician put it in I would feel confident it is grounded and properly installed.
You can pull the recep out without taking the wire off and check if it will make you feel better. Not to imply you're not bright,(its a habit working with new guys) but turn the breaker off first!!
My next concern would be the amp draw of the compressor on number 10 wire. Number 10 is good for 30 amps. Check the nameplate of your compressor motor and it should have the amperage on it.
Other than that ... you should be good to go with no worries.