What you are looking for is a dip in voltage when you start coasting to a stop light etc.

I've had 3-4 e53 Alternator fails and every time I could see the voltage dip to say 12.5 right after I lift off throttle from say 35 mph.

If you had an oil leak issue that wetted the alternator the odds are nearly 100%.

That said, I've refurbished a couple valeo models and it's not difficult for $20.

Graph the voltage or if you have a good DMM, record min. /max. The dip or peak could be milliseconds, you won't see but the computer will.

__________________
2011 E70 • N55 (me)
2012 E70 • N63 (wife)