Sandpaper will do fine. If the pulley is polished, which is really a judgment call requiring some comparative experience, the belt will be glazed. A fingernail pressed into the side of the belt that makes contact with the pulley should find it somewhat pliable, not hard and crisp -- but again those are words that defy long distance communication. It is most helpful just to know the silent slipping is possible.

To check the harmonic balancer, you'll take some paint or white-out, and draw a line from near the center, across the rubber isolator to the outside of the crank pulley. Then after some few days or a good drive, check the line hasn't become separated.

Probably the best way to determine if the system is slipping, is to monitor the charging voltage while idling. If it sinks below 13.5 much, spritz a bit of water on the alternator belts to see if it comes back up then.

It can still drive you nuts, misbehaving only when it is warmed up, or become randomly intermittent; hence my suggestion to get a voltmeter you can watch while driving. Otherwise, if it doesn't charge idling in the driveway, you'll be able to fix it for sure. Either way, you need a means to monitor the voltage.

Someone, recently, connected their exciter wire to the wrong tab on the alternator, so if you've installed this one, make sure it is on the D+ terminal and not on the W terminal.
--
Art Benstein near Baltimore

The roundest knight at King Arthur's round table was Sir Cumference.