Different ways you might notice you have a failing/failed alternator
1st case of alternator failure:
My '93 Legacy L Wagon was having trouble keeping a charge in the 5-year old battery. I took the car to a national chain parts store for a battery test. It showed "excessive ripple" but the guy who did the test did not know how strongly that condition suggests an alternator problem, and I didn't know anything about excessive ripple either. I figured it was time to replace the battery anyway. After installing the new battery I took a 70 mile round trip with the AC & headlights on. After returning home and re-starting the car and driving a few blocks, it suddenly lost all electrics - no spark, accesories, etc. I got a jump start and the car limped the 4 blocks home with all accessories turned off, just running the spark plugs with the last juice left in the battery. I replaced the original 18-year-old alternator and all was well. I believe the new remanufactured alternator I purchased was a Beck/Arnley.
Checking around the forums I learned that excessive ripple might be a sign of a failing alternator. There are several diodes in the alternator, and if 1 fails, the alternator still charges the battery, but has lost a third of it's charging ability. Successive diode failure prevents the alternator from keeping even a partial charge in the battery. Does that sound correct? Anyway, I had no warning from dash lights in that failure, just low charge to the battery.
2nd case of alternator failure:
Fast forward 18 months, same car. Driving across town I heard the radio cutting on and off, engine stumbling, and aggravated by use of turn signals. I was near an auto shop and my car limped into their parking lot where it stalled, and would not turn over. Right away they suspected the alternator. I didn't think this was an automatic diagnosis because the alternator was only 18 months old. I was thinking it must be some serious problem, but they were right, it was the alternator. Unfortunately, the alternator I had a friend install was from a reputable locally owned auto parts store, but the warranty was only 1 year for walk-in trade vs. longer for their commercial customers (auto repair shops.) The mechanic where my car stalled called around to find an alternator in stock. The only place was OReilly. They delivered one within the hour and I was on the road again.
3rd case of alternator failure:
About 3 weeks after having the OReilly re-manufactured alternator installed, I saw the battery and brake lights coming on at the same time. This would happen intermittently over a period of a couple of days before I searched the web and learned this is possibly symptomatic of a bad rectifier in the alternator. I took the car to the same shop on a Friday afternoon, replacement alternator to be installed Monday. The owner even gave me his card with his cell number, saying if the car broke down over the weekend I should call him because the warranty would cover towing to his shop. I used my charger to keep the battery charged until my shop appointment. The alternator was still under OReilly's full warranty covering parts and labor. I wasn't sure I even wanted another re-manufactured alternator after all this, so I stopped at the Subaru dealer to see about the alternator recall I read about. The parts man was not aware of the recall, but further reading tells me the recall did not apply to '93 models, so maybe that's why nothing showed up for recalls. I asked what an OEM new alternator would cost, and I believe the price he quoted me was over $400.
Well I thought I was out of the woods once the 3rd aftermarket alternator was installed, but a few miles after the installation, I lost power steering, and again the battery warning light came on. I drove right back to the shop before even popping the hood. When I got there and looked inside, the belt tensioner bolt had snapped off. They fixed that, but later, when I looked at how the tensioning system works, I suspect they had tensioned the belt with the long tensioner bolt and then failed to clamp down the tensioner, so all of the force was on the long threaded belt tensioner bolt.
I'm at the point where I feel like I should just carry a a spare alternator in the car to be ready for the next failure.
I hope this helps anyone who is having any of these alternator symptoms in their Subarus, and this is just my personal experience - I'm by no means a mechanic or car enthusiast, just an average owner who loves my Subaru, so check with someone knowledgeable before you go by what I've suggested.