An engine should be properly diagnosed before it is disassembled for two reasons. First, to determine that a repair is really necessary. And, secondly to diagnosis the exact location of the problem while the engine is still intact.
Normally, main bearing inspection requires removal of the oil pan. refer to your vehicle's service manual for further instructions.
As shown below, bearings can fail for a variety of reasons. Oil starvation and dirt are the major reasons for bearing failure. Problems in other engine components, such as bent or twisted crankshafts or connecting rods, or out-of-shape journals, can also cause bearings to wear irregularly.
Inspect your bearings for these conditions.
Common forms of bearing distress. Courtesy of Clevite Engine Parts.
A loose crankshaft main bearing produces a dull, steady knock, while a loose crankshaft thrust bearing produces a heavy thump at irregular intervals. The thrust bearing noise might only be audible on very hard acceleration. Both of these bearing noises are usually caused by worn bearings or crankshaft journals. To correct the problem, replace the bearings or crankshaft.