With the plug pulled from teh housing stick your pinky in the hole and wrap it down. Level should be a little below the fill hole. The exact amount of fluid isn't that big a deal as long as there is enough. Too little is bad. Should be, roughly, about 3 quarts though in an 8.8 rear end.
The plug on the 8.8's is located half way up on the left side of the housing towards the front. It's right above the, I guess you could call it a "support rib". Just behind where the "dog bone" bolts on to the bottom of the axle. You'll need a 3/8 drive rachet and a 6" extension. Check it when it's hot after you've taken it out for a drive. The fluid level should be right up to the bottom of the threads of the inspection hole. If a little comes out, it's o.k. It's also o.k. to check it with just the rear of the car up on ramps or jack stands. You don't need to have the car car level with all 4 tires up off the ground equally. I just did mine 2 months ago. 2 quarts and 4oz of friction modifier was plenty. Unless you plan on cracking the cover and replacing the gasket, you might be o.k or you might just need to add a tad.