It's difficult to measure the glucose produced because it is quickly made into starch - you may already have put pieces of foil onto geranium leaves to show this?

Because measuring the glucose is difficult, how about measuring the amount of carbon dioxide used up? That would give you a clue.

The classic experiment, though, is to use the amount of oxygen produced as a measure of the rate of photosynthesis. When I was in school we used pondweed (Elodea) and put it in an inverted beaker in good light. The bubbles given off are oxygen and we measured the volume.

It's not perfect, but it'll give good approximations.