Voltage is the only way to guess at the pack capacity without the use of a discharger.. The higher the voltage compared to similar packs the greater the charge left in it.
Packs can vary in labled capacity by a little, so this method would be ok for comparing 4200 adn 4600 together but not 4600's to 3300's./
NiMH cells can sit stable up to 1.4V per cell after the "surface" charge fades.
So if you are checking 4-cell packs and you have one at 5.34V, 5.12V and 5.48V, Your best bet is the 5.48V pack since it will be the most full pack compared to the other packs.