For arguments sake, lets say your Charger can charge at 10A max.

An 0R02 Resistor would drop 0.2v at 10A charge current dissipating 2W. An MSR-3 (3W) or MSR-5 (5W) type Resistor is perfect. Fit it in the NEGATIVE supply lead to your Battery. If you now set 0.2v as +VREF (a couple of 1% Resistors as a Voltage Divider from your +5VDD Supply) then your ADC will be able to measure (across this Resistor) from 10A Charge current right down to just under 10mA (10A/1024 ie steps of 9.7mA). You may need to check if your chosen ADC's VREF can go that low).

There are other ways, but hey, 50 cents for three precision Resistors can’t be beat. Sure you might need some over-current /short-circuit protection at the ADC input, but that’s the easy bit.

You can do the same thing with a PICs Comparators, but that will just give you a threshold under which you can say it's not doing much charging.