You can do it on the wires/pins coming from the plug. Right there where the plug is soldered to the board.
Or you can take the measurments from the plug itself, after unplugging the bms from the battery. Like this.
Either way, you touch two wires, and march both probes down the line to get each cell without having to do subtractions, like you would if one probe stayed on the end wire.
THE LIPO RULES. NEVER ABOVE 4.3V NEVER BELOW 2.7V DON'T PUNCTURE
Ideal charging /discharging range for Lipo, 3.65v minimum 4.1v maximum
See battery technology section, FAQ thread at the top of the page for lipo noob info.