Front Wheel Bearings. How do you know when bad?

[remist17]

I have a 93 Silverado 4x4 that has a good nosie coming from the front end. Noise changed with speed. Hav enot had a chance to see if it happens in 4x4. In 2 wheel drive the noise seems to have gotten louder at 55 (cruze speed). The funny thing is when its cold and sits for a while it takes some time to hear it. I am thinking it needs to warm up? I am thinking its either the dif bearings or wheel bearings. I do not know how to check to see if the wheel bearings are bad. Can any one tell me? I am hoping I can just repalce the wheel bearings in stead of getting the front differential worked on !!

Thanks