Forum member HKJ probably has a better answer, but being an EE, what needs to happen is to use ohm's law. V/I = R. Since you can't actually put a DVM on the resistance that is inside the battery, you can measure the voltage at the terminal of the battery before putting any current into the battery and then measure the voltage at the battery while charging. It will be higher. The voltage at the battery when charging has to be higher than the actually battery voltage to over come the internal resistance so that current will flow into the battery.
That difference divided by the charge current is the internal resistance.
Vbattery (when charging) - Vbattery (when not charging) / charge current = battery resistance.
Note that the battery voltage will be increasing when charging so you need to be prepared to make that measurement the moment you start charging.
Here is a diagram:
The 1 ohm resistor is put there just so you can measure the current. If you have another way of measuring the current then the 1 ohm resistor isn't needed, such as a good power supply with accurate meter or even another DVM in series with the battery. Voltage measurements need to be made right at the battery terminals and not any distance from the battery or you'll be picking up resistance of the wire. Power supply voltage of 4.20v doesn't need be exact........just a value that will cause a decent current to flow.
Edit: I just looked over HKJ's link and he has a couple of different examples worth looking over. He's showing how to measure using current flowing out of the battery whereas I did it using current flowing into the battery.
