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Yeah and the plug will be a socket not a bolt head.
Usually a 3/8 square drive ratchet will fit. It may take a lot of torque to get it loose. I like to use a 1/2 breaker with a 3/8 adapter. Use good tools. I have snapped off a cheap 3/8 doing this.
Once the plug is out stick your pinkie in the hole and if you fell fluid you are good.
Most of us add fluid till it just dribbles out the hole.
Be sure you know what kind of fluid you need. Many use ATF not gear lube.
I don't recall ever seeing an axle call for ATF...
Had an 86 Tbird used ATF in tranny and diff. Read of others but don't recall since I had no interest. I was rather surprised, but that's what the manual said. Loved that car.
Usually light duty vehicles using ATF. I think it was to squeeze a little more EPA rating for marketing purposes.
The key thought is "Make sure you know what fliud to use".