Like everyone else indicated, it really depends on when you are adding supplements, and if you are doing manual dosing or if you have a auto dosing unit.
If dosing by hand, like Jgao said, test before dosing, and about an hour after to see the change.
If using a dosing unit, depending on how many times its set up to dose per day, I would test before it doses and about an hour after still. You could also do the same test morning and night if you want to be extra sure its stable. I have my dosing pumps run 12 times a day to try to keep everything run as stable as possible.
- David