Hey Robi, yes, getting a voltmeter is a step in the right direction! Some of the guys I fly with like to have lots of margin, as Figment mentioned and stop flying when they get to 5V on a 4 cell pack! That means 2 flight with some models with lots of servos! I'd like to suggest a method which will take a little of the guess work out. But it will take a discharger that measures time. Futaba has one that is "only" $40.

First charge overnight or whatever you do normally then discharge making note of the time. Next, charge the same way and remember the number of flights you made, then when you get home discharge again. This will give you an idea of how much battery power that particular plane uses. If you make it a habit to voltage check before each flight you will get to know the charateristics of that pack/plane and will probably be able to catch a battery going bad before it does any damage. If you periodically discharge the packs with the discharger, this is also another good way to catch a pack going bad.

Seems like a bit of work, but gives me piece of mind, especially since a voltage check by itself is not a perfect indicator of charge.

Good luck!