You can determine online how much money is on them. A couple years ago they changed the card type so it depends how old your cards are, if they are still useable. If they are still good, I would take the one with the most money and use that one or add money if you intend on riding the metro. I kind of doubt you can get money back.

I have a bunch of fare cards with one or two dollars on them (I really don't know since I haven't checked at a kiosk). Can I trade them in? Right now, they are sort of like Amazon or Starbucks gift cards that you'll never redeem.