Run the battery down and see how much you can put back in.

you can only see how much the charger puts INTO the lipo, i.e. charge it up and it puts in 4700 that means it had 300 in it (just an example), it wont tell you whats there. Someone should make something that does though. well I guess thats where you go by voltage

yeah i think its hard to accurately determine, but if you run it down to cutoff first then charge, you can get a better indication. still not perfect because chargers arent 100% efficient. the only real way is to have it charged then run it down to the cutoff, and measure the output of the discharge.

It will always vary a bit from setup to setup and charger to charger. What you have the LVC set at will effect how much you can put back into the pack. A conservative (high) LVC setting will prevent getting the last drop out of your pack, but it will also help the pack last longer. Most damage is done to lipos in the last minute of discharge right before it dumps and it has a harder time holding voltage. I try and observe the rule of I dont want to run more than 85% of the juice out of my packs. I set the LVC a bit higher so when I hit the LVC on a 5000mah pack I would want to see the charger put back only around 4250mah or so and a 4350 pack only takes back 3700mah or so to keep it safe and make them last. Usually I find that the resting voltage is somewhere around 3.5v when they are at 85% discharged. Of course that will vary from cell type to cell type.

Being conservative with your LVC settings will help make your lipo investment last much longer and the minute or 2 you give up on runtime is worth it IMO.

Also, if it is a damaged lipo or old it will have less capacity than when it was new.