the black one will be negative. or for ones like a AA battery holder, then whatever is connected to the negative end of a battery
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if orksecurity didn't answer the question perfectly (which he basically did) you could use a multimeter to test the polarity of the battery charger. and if you're really ambitious you could take the whole thing apart and look at the wires connecting to it (red positive and black negative) and if you're super ambitious you could take the whole thing apart and separate the cables out till they reach the wall socket the round hole being a ground (negative) the left square hole being neutral and the right square hole being positive.
Depends on the kind of battery, of course.
The simplest way to answer this for yourself is to look at a battery -- most of them are labeled for which side is positive or negative -- then look at how the battery snaps into the holder. That usually makes the answer obvious, unless (as in the case of a 9V battery clip) the leads are hidden under something like a plastic cover. Very often you can still tell which lead is coming from which side if you straighten them out and gently tug on them.
If that doesn't help you, you might want to make the question a bit more specific.