Its not diff - on the back of the balance conn, you can see metal part of the wire going into the connector. Touch two of these metal pieces that are side by side - thats one cell. I started on the side with the black wire. Move both test probes over by one wire and its the 2nd cell - you will use one of the places you touch again, but with diff prob.

If you get a neg voltage, you've got the polarity reversed, swap and it will show up correctly.

I don't have a 2S, but I have a 3S Align lipo (heli) and it has 4 wires, so to read 3 cells, you have to reuse some of the wires in the conn.

Experiment, you can't hurt the lipo as long as you don't let your probes contact each other and short a cell.