A drop checker is a nice visual guide as to whether the tank is absorbing CO2 and releasing it at the right times but is pretty inadequate to tell exactly how much has been absorbed. With time you will get used to the color changes and it will give you insight into what's happening with the CO2. For instance when I get home from work I glance at the drop checker and if it is not the same color of light green I'm used to then I'll check the bubble counter and pressure gauges to see what has changed. In the morning I expect it to be back at its normal blue color.
As far as to whether your gassing your fish this is best seen in their behavior. If they are at the surface and their gills appear to be working hard then you've got too much CO2. In the afternoon when the CO2 is at is peak mine will remain still with little movement but they do not appear stressed at all. When the CO2 shuts down and the air pump starts an hour later then they are quite lively again. If I add a new fish during the peak CO2 times it will stick to the surface and appear to pant. Next day though it appears to have become acclimated and no longer hangs out at the surface.
Keep your eye on your fishes behavior, the color of your drop checker, and the speed of your bubbles in your counter and you will soon get the feel of how to judge your CO2 levels. You can measure the PH and the GH and compare these to a CO2 chart but if you are like most of us that is way to much effort to do on any sort of regular basis.