Hi,
measuring SG means measuring specific gravity (also called relative density) of the battery electrolyte. This can only be done on a flooded wet cell battery, ie one that has vent caps. Basically, what happens is you take off the vent caps and use a hydrometer to measure the strength of acid/ state of charge in each cell (all 6) of the battery. The hydrometer draws some of the acid into its tube and you can see how much charge the battery has by looking at the floats on the hydrometer scale.
But really, I wouldn't bother with this quite yet. I would first assume that the charge rate on the charger you speak of is way too slow. I agree with the others on this forum. If you are sure your charger is 70 milliamps, ya it will take days maybe weeks to charge. You are better off with at least a 1.25a battery tender or something else with more current output.