Hi,
This is quite straightforward. Those cells you refer to usually have a ma-hour rating printed on them. So, you discharge the cell thru' a known resistance and measure the time it took for the cell to get to a point where it would be considered 'flat (around 3 volts for this cell)'. This would give you the actual ma-hour rating of the cell, as against the manufacturer specified one (current drawn*time taken in hours).
Regards,
Anand
Added after 7 minutes:
Forgot to mention, the voltage readings you took seem to be without a load; these are bound to be misleading and not indicative of the health of the cell.
Regards,
Anand