If it was the rod bearings, you would probably hear a tapping noise as if there was a bolt floating around loosely inside the engine but the noise would be because one or more of the connecting rods was knocking against another due to play in the bearing.
If it was a main bearing it would be a constant oil pressure drop and would have dropped from a normal of maybe 60 psi cruising down to 10. Not just 10-15 at idle.
You can also loose pressure through the camshaft or hydraulic lifters.
There really is no way to tell for sure without dropping the oil pan and opening and examining each and every bearing.
IF they all looked ok at that point then you can measure the clearances using "plasti gauge".
PROBABLY if you have a bad bearing, probably, when you take off the bearing retaining cap, you will immediately see the condition of the bearing and of the race on the crankshaft.
I have seen bearings changed in the car but it isn't easy to do at all and it would be questionable as to how long they were going to last.
It could essentially "self destruct" almost immediately or could be ok for sometime.
Usually it is just the rod bearings that survive being done in the car. I don't know how you would do the complete main bearing without dropping out the crankshaft.
For that it would be better to pull the engine and work like a civilized human?
How many miles are on the engine and do you know it's entire history?