How would you tell if my truck needed new front wheel bearings?

If no noise, raise the tires off the ground, grab the tire at 12 & 6 and try to move the tire/wheel wiggle by pushing in and out to see if there is any movement. I f no movement and no noise, they're probably OK.

useally they make a low roaring noise when driving. I have seen it come on gradually and the driver of car may not notice it until it gets much louder.

you may not hear them as the noise came on slowly,but a pro would pic up on it.call him and ask why he stated that