How to test internal resistance on small lithium polymer cell?
Measure open-circuit voltage \$V_{oc}\$, then load the battery with resistor \$R_{load}\$ and measure voltage \$V_{loaded}\$.

$$ R_{in} = R_{load} \cdot \frac{V_{oc}-V_{loaded}}{V_{loaded}} $$

This is the internal resistance at DC and at your specified load.  Internal resistance varies with load and temperature and battery charge and age, etc.  And you'll need more advanced equipment to measure at higher frequencies.