I think honestly, the best answer is to let the cells rest awhile and then measure the open circuit voltage with a voltmeter. With every NiMH cell I own I can calibrate the voltage with the state of charge once I am familiar with the characteristics of that kind of cell. Nickel zinc cells have characteristics closer to Li-ion (CC/CV charge algorithm), so open circuit voltage should be an even better indication. Just measure a few cells that are fully charged, 3/4 charged, 1/2 charged, 1/4 charged, etc. and get a feel for what voltage they have.