I tried that method when my bearing spun, nothing showed up in the oil, I even tore apart the oil filter and it was clean. It doesnt work all the time. Sure as hell had flakes hanging around in the oil pan and heads though
the bearings aren't aluminum though. Most bearings are made up of layers of steel, copper, and something else that I can't remember right now. But the majority is steel.
Compression check won't find a bearing problem. About the only way to test for it in the early stages (before it starts knocking) without tearing into the engine, is to have an oil analysis done. But you can pretty much bet that if it won't hold oil pressure, the bearings are on their way out.