Any meter reading a voltage direct from a battery will invariably show you an indicated off load voltage... and thats it! It will never tell you how much current capacity is left in the pack. Using a water analogy, you know there is water in a tank because it comes out of the tap but you can't see in it so you don't know how much. Different batteries behave differently during discharge, for instance, nicads will keep going and then drop off suddenly whereas dry cells will slowly keel over from the moment you start using them. A battery's vlotage is best checked when it is being put to work and current is being drawn, then the voltage will drop and if the thing is virtually flat the voltage will start dropping off fairly rapidly. Another indication might also be the voltage recovery time when the load is removed but as with all things, a bit of experience with meters and batteries is usually called for before any of it gives you a feel for whats going on.
If you have doubts about a battery's condition this can easily be verified by first knowing what your boats maximum current drain was when you first installed your new fully charged battery, theoretical run time should have been calculated against the battery's current capacity. An initial run on the pond can be used to verify this and may actually exceed it. When run times begin to fall off, a quick current reading at max throttle will tell you if the current has increased due to tight proshaft bearings, faulty motor etc and if they are much the same as they were when you first tested them, your battery is probably stuffed so it's off to Maplins with you!
Current readings should be considered the God of the model boater, they tell you all you need to know about the health and efficiency of your setup, it can prevent you from burning out motors prematurely, wasting battery run times and cooking speed controllers.... and stop you wasting money on a battery you didn't need. This is so very true for the model submariners where we consider minimum current drain to be the ultimate goal and I think most of us are familiar with current readings... or should be!