How can you tell if a wheel hub bearing for a 99 explorer is bad?

How can you tell if a wheel hub bearing for a 99 explorer is bad?
There are two main things to look for: noise, and end play.

Drive the vehicle at a steady speed and gently rock the steering wheel side to side causing the body to "roll" and shift weight between the left and right sides. 90% of the time the noise will get louder when the bearing has more weight on it. I've had cars that didn't fit this rule, but in most cases both bearings were bad to begin with.

Then, raise the vehicle off the ground, grab the tire, and see if you can feel it move in and out, up and down, etc. The only movement a tire should make by hand ever is rotational.

Because most unitized bearing assemblies only have 1 bearing internally, not an inner and outer anymore, they usually only exhibit excessive end play when they are REALLY worn out. The noise usually shows up first.