So, after an epic hike through the nether, I'm fairly sure no one is near me, but the map thing is just black where I am. Is there another way to check to see if anyone else is within 3k blocks?
There's no way to check if players are near you. Your best bet is to attempt to get the outpost established and see where that goes.
You're safe if the area all around you is black. Black areas on the map mean that nobody has built there before.