This is a blown up image showing the connector side of the flex cable coming from the battery.
On it you will see the 3 copper coloured connectors in the plug, with what I think is the temperature connector in the middle. By the physical size of the connectors the +ve and the -ve connectors should be the 2 outside ones of the three and using a Voltmeter it shouldn't take much to work out which is which,
click on image to enlarge for better viewing.
Test procedure using DMM -Voltmeter - 20V DC scale
Place Voltmeter red test lead on one outside connector terminal of the battery cable and the Voltmeter black test lead on the other outside connector terminal of the battery cable.
If the Voltmeter has a -ve symbol in front of the voltage reading then the red lead is on the -ve terminal of the battery.
If the Voltmeter shows only the voltage reading then the red lead is on the +ve terminal of the battery.
Ensure that you do not bridge across the terminals with the meter test lead probes when measuring the voltage, as you could s/c the battery.