How do I know whether or not I've got good or bad bearings?

You can try to find the Abec rating of the bearings. If you look closely enough on the bearing itself, it should have a number. 5 is the lowest rated bearing, although there are unrated ones.

The abec rating is just a rating on how smooth the ball bearings inside the bearing are. The higher the numebr the smoother the bearing. If you maintain your bearings though, and Abec-5 will easily outlast an Abec-7 that's not maintained

Basically: Research your board, see if you can find the abec rating of the bearings, if not, look on all the bearings very carefully themselves (it's usually on the side). If you find no number, they are either not rated, or are ceramic or swiss.

If they are ceramic or swiss, then whoever said they're bad is a liar

Also, most stock bearings are usually Abec-5

You'll find out though once you ride if you need new ones or if you just need to lube them up. If you feel like your board can go faster but it won't, they just need lube.

you will find out when u get them.