Estimating Li-Ion (or any other type) battery resistance
Generally to determine the resistance the cell in question is given a known load, and the voltage across the battery is measured. Comparing the loaded to the unloaded readings the cell's resistance can be calculated. 

Generally these tests are done with a DC load but in cases where you need more in depth information AC load tests can be done in a similar manner to determine the overall cell impedance.

This battery university article has some more information on the subject: http://batteryuniversity.com/learn/article/how_to_measure_internal_resistance

That site has tons of useful knowledge on batteries in general, definitely worth going through.