How to clean wheel bearings??

The inner race (behind the rollers) sees the highest loads (two convex surfaces) and the highest speeds (smaller diameter), it's also where you will most likely find pitting and it's the most difficult area to inspect and frequently not looked at.
Clean them, dry them, inspect them in good light, re-pack them. It's basic maintenance, there are no good short cuts.
If you want to replace them every time go ahead, your still gonna get your hands greasy packing the new ones, properly maintained they will go a million miles.
My dos centavos.