how does a crank shaft bearing go out even if there is oil in the motor

Lack of oil pressure, lugging the motor can cause loss of oil film strength, and essentially it needs oil pressure not just oil presense.
Cool.
Buddha.

how do you check to see how much oil pressure you have

You can run a mechanical guage tapping into the oil galley plug under your right toe as you're riding it, or run a electrical guage by getting into the round cover and replacing the stock sending unit (the on on the bottom with 1 wire comming from it) with a true sender that sends a voltage and a guage that reads it be fitted somewhere.
Cool.
Buddha.