I use my tds meter for water change water. Whatever remineralizer you're using will hit a corresponding tds for gh values.
A meter for this actually does exist, it uses ion selective electrodes. You would need to get a divalent one for measuring gh. Probably very expensive, and not worth it for our hobby.
Last edited by xmas_one; 11-19-2013 at 05:33 PM.
Reason: found a meter