Are the terminals on the battery not marked? Seems like they should be! The best way to find out is to get a simple handheld test meter and test for polarity. You can get some really cheap ones at Harbor Freight, or usually the chain auto stores such as Advance Auto or Autozone have some. Radio Shack may have a cheap one too. Or you could borrow one from a friend.
Once you get the meter, put the red lead in the hole marked (+) pos. Plug the black lead in the hole marked (-) neg. (NOTE: there may be another set of holes to measure current with, you won't use them for this measurement (read the users manual if handy!)
Once you get the leads properly hooked up to the meter, set it to read DC volts, on a scale setting that is at least 10 VDC full scale. Put the red lead on one battery terminal, and the black lead on the other one. If the meter indicates a negative measurement (indicated by a (-) sign on digital meters or the needle trying to go the wrong way on analog meters), then reverse the leads (on the terminals, not the meter). You should now get a positive reading (indicated by (+) on a digital meter or the needle giving you an indication of how many volts it is measuring on an analog meter. At this point, the meter should indicate somewhere close to 6VDC on the battery.
Once you get a positive reading on the meter, then the red lead will be showing you the positive terminal of the battery, and the black lead will be showing you the negative terminal on the battery. But, this will be true only if you have the leads hooked up to the meter correctly!! That is very important!
I suggest once you get it figured out, mark the terminals clearly so you (or anyone who comes behind you) won't have to go through it again. Oh yeah, in case you didn't realize it, you need to disconnect the leads from the battery before charging it. That's always the smart/safe thing to do. You can leave them on, but if there is something wrong with the wiring/electronics that is shorting the battery to ground, you may not get it fully charged.
Good luck!