If you connect a voltmeter across the battery, it should read c.12.3V with the engine off.
Start the engine and put some fast idle on it (1500 - 2000 rpm) and the volts should read c14.6V. If yours is less, then the alternator isn't 100%.
Switch the lights on with the car idling and get someone to prod the gas pedal so the revs go to 1500-2000 for a second and then drop back down to idle again. If the lights get brighter with the extra revs then it's more likely the regulator/rectifier than the alternator itself. If the lights get brighter but not much... and the battery voltage is only slightly higher than 12.3V... then there may be a problem with the alternator itself (brushes usually).
If there's no difference to the battery voltage or the lights, whether the engine is at idle or at fast idle then the alternator windings are most likely knacked.
None of the options above is better than any other. You still have to remove the alternator to fix it and the rectifier/regulator will probably be more expensive than buying a whole new "exchange" alternator (cleaned up like new but "second-hand" frame and parts... fitted with new windings, brushes and regulator/rectifier).
Don't buy a new alternator without checking the price of a refurbished/exchange one... but don't fit something scabby taken off a breaker... it should look like new (unless you really enjoy dismantling alternators, of course..
)
Ralf S.