I just bought a used DS lite, and I was just wondering how I can find out if the warranty on it is still active
http://www.nintendo.com/consumer/manuals/warrantytext_eng.jsp The DS has a hardware warranty of 12 months from the date of purchase. You'll need to have proof of when it was originally purchased, hopefully the person you bought it from still has a receipt. But the warranty says "Original Purchaser" so you may not qualify for it since you bought it second hand.
I say the original warranty has expired, but you can try to register it at nintendo.com and see what happens. Did this second hand shop offer a warranty?