When the output of your stock alternator can not keep up with the current draw of the accessories that you normally run. A good battery will help but it is only a buffer between the alternator and your accessories so if they are accessories that you run simultaneously for long periods of time, you need to consider upgrading.
For example, your '98 may have a 124 amp alt from the factory (not sure how common it is, but my '99 did). Thats actually a pretty decent sized alt. But a winch can draw 300+ amps under a full load. So the alt can't supply the full amperage the winch is drawing, which is why a good, possibly oversized or dual, battery setup helps. It supplies the extra power needed, then once the load goes away, it re-charges the battery.
So assuming you aren't getting obvious signs that your alt isn't cutting it (dimming lights, trouble starting if the jeep hasn't been running for a while, or something else), you need to figure out how much power the accessories you are adding are going to draw when running. Amps = Watts / Volts. For calculations I usually use 12 volts even though while running you will usually see 13-14. So for a pair of 55W lights,
55 W x2 = 110 Watts
110W / 12 = 9.17 Amps
Some accessories may actually have their amp draw listed and you can just use that. I think some compressors are marked this way.
So start totalling up your accessories and see how much load you are adding. There are estimates floating around for what the vehicle consumes on its own but I don't remember off the top of my head.
Also, depending on how much extra capacity you need there are fairly straight-forward 136 amp and 160 amp alternators that will bolt in with minor modifications, which would give you added output without breaking the bank.
Edit: Also, if you are upgrading the alternator, you should be upgrading the wiring as well!