All of the prior suggestions about checking output are good -- and also check your belts (I find mine needed a little tightening in this sudden cold) -- but I just wanted to add that, if you find that the alternator isn't up to par, don't pay big bucks to exchange your alternator just yet.

On both of my '93's, one at 145,000 mi and the other at 190,000 (suggesting that this point in the mileage is unpredictable), I found that the brushes had worn down to where the alternator exhibited some bad behavior. But instead of the expense of swapping the whole alternator, I just replaced the brush/regulator set.

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I've already removed my preheater "stove" (the sheet metal around the exhaust manifold) and duct to the air filter box for other reasons, and with those out of the way I am able to work from above and swap the whole brush and voltage regulator set with the alternator still in place (I don't even have to loosen the belt to move it even a smidgeon), the only tool being a small mirror and a phillips screwdriver (it takes about five or ten minutes the first time, and faster after you've learned to get a feel for compressing the brushes' springs. If you're keeping the preheater system, it might be harder for you, though, and you may have to work underneath, instead.

Hope this helped.