are there any tell tail signs of when the drive shaft bearings fail? if that what their called..

talkin' about these babies:

thanks for the pic paul1985!

little back story;
couple months back driving along and my wheel goes all wobbily, think damn must have a flat tyre.
pull over to a side street, all tyres are fine and properly inflated.
get back in and move slowly, seemed that with every 360o rotation of the wheels i'd hear a knock coming from the front coupled with the sensation of a wobbling steering wheel.
baby it about 50m down the road and the wobbily steering wheel stops and the sound is no longer there.

takes it home and throw it on stands for inspection and nothing seems out of place, nothing seem missing and theres no extra bits which shouldn't be there.
took out the drive shafts and had a look at them, appears to be visually fine on the outside so i pop them back in.
check all my suspension components front and rear and everything seems to be in order, bolted up and nothing out of place.

now it's been fine ever since, no more knocking sound but i do get the occasional steering wheel wobble sensation for a sec.
generally happens around the 40-60km/h mark while accelerating in second or third and sometimes at high speeds ~110km/h while travelling on the freeway.

could this be a sign of a failed drive shaft bearing?
or something else more or less serious?
not sure on the age of the current ones, should i just rip off the boot and replace them anyway?