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Alternators do not slowly drop in voltage output as they wear. They are either producing voltage or they aren't. 13.5 - 14 volts at the battery is perfectly within spec. There will be a voltage drop between the alternator terminal and the battery. 1/2 a volt drop is possible. Voltage output should be measured right at the alternator output terminal to eliminate the voltage drop from the true output. Anything over 13.2 volts is sufficient, really.
Alternators use brushes as part of the field coil. The brushes are subjected to constant friction because they are contacting the spinning slip ring. Eventually the brushes wear out, the field coil goes open circuit, and the alternator can no longer function.
To determine the remaining life of an alternator, it must be disassembled. Measure the brush length and compare them to the minimum length specified in the factory service manual. (6mm on an S14)
Few things can go wrong with alternators. Worn brushes is the most common issue. Bearings can fail and become noisy. Oil or coolant being sprayed into the alternator can interrupt the field coil circuit. Cars left sitting for extended periods of time can accumulate corrosion in the field coil circuit.
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