Help with battery charger

First mark one of the charger wires to identify it. Now wire up the lamp between (in series with) the marked wire and the battery +. (i.e. connect the marked wire to one terminal of the lamp, then the wire scrap from the other lamp terminal to the battery + ).

Now with the charger TURNED OFF, take the unmarked charger wire and touch it to the battery negative post. If the bulb stays dark, the marked wire is the charger +. However if the bulb lights (it may be somewhat dim), the marked wire is the charger negative.

Once you've figured out the polarity take some red tape or red paint and permanently mark the charger + wire

You weren't thinkin' of the old "touch the leads to a battery with the charger shut off, and if it sparks it's backwards trick" were ya?

Digital Multimeter... if red and black are correct, will read XX Volts. If red and black are not correct, will read -XX Volts.

Analog multimeter... if red and black are correct meter will swing upscale to voltage reading. If red and black are not correct, meter will deflect downward. (Quickly disconnect leads if reverse polarity is so indicated.)

When you have matched the red and black leads from the meter with the charger leads so as to give a positive voltage reading, install your red and black charger clamps to match the color of the meter probes.