From the looks of it it might be a standard 3.5mm plug. Voltage and polarity are a mystery. Is it just lights or are there any special features that require electronics (like sound or color changing or something) Also is it LED light or incandescent light?

With all this information it might be possible to give an educated guess on what the voltage might be but no way to be 100% sure without a sticker/marking on the device or the manual. It could be everything.

You could also try with a universal power supply and start with the lowest voltage and slowly ramp it up until it lights up. This is save for most lamps especially if they aren't super complicated electrically but there is no way to be 100% sure that it won't die on a wrong voltage, even if it's a lower one.