mostly empty it would read More than ~6v if its still working right.
max discharge on it would be ~5V most cams wont take it down that far, and it should bouch back up some when the load is removed.
when fully charged it should be reading close to 8.4V
like labrat said, and from what it already sounds, if it isnt running your device well it is probably to aged, out of balance or just finished. even if it shows good voltage, if it doesnt charge, self discharges, or doesnt hold good capacity (have runtime) not only is it dead, but its not concidered totally safe Either.
charge it, run it down, charge it again if it aint working , its dead jim.
potentially you can change the cells and restore it to better than new, but some curcuits placed on them make that harder than it looks.