Maximum current and voltage of Small Ceramic Capacitors?
ceramic through-hole caps are usually rather high in voltage rating, i don't think i've seen one in person under 25V except maybe those with high capacitance, >=1uF and they are usually 16V although i know some with smaller voltage ratings are available.

A simple subjective measure is size.  For a given capacitance and type of construction (ceramic in this case) the smaller the package the lower the voltage rating.

This rule also means that if you can find a comparable part on digikey to what you have in hand, the voltage rating is probably the same, there aren't too many manufacturers tricks left in the bag when it comes to through-hole ceramics.  Of course the only way to be sure is to try it.

As for current.  Ceramic capacitors are rarely, if ever, specified with a maximum current.  If the current flowing through the cap is large enough to matter your problem will most likely be the impact of the series resistance of the cap on your circuit or it generating enough heat to push the cap outside its thermal limits. 

If you need assorted capacitors for your lab i'd look at the cap kits on digikey, there are a bunch of them and they usually come in little "fishing box" like containers that keep them all sorted for you.  You also gain the advantage of knowing who made the caps and get actual specs which isn't true of the assortment packs at places like radioshack.  You could also get one of those slide bin storage things from a hardware store and make your own capacitor assortment.