If they start gasping at the surface, you can do a water change. Also increasing surface agitation, with an air stone etc, will help to gas off some CO2. You'd prob have to have a lot of DIY Co2 to bother the fish during the day, or a very low plant mass, or lights that are too low so the plants can't use the Co2. The time when you need to worry about too much Co2 being in the tank is at night.
If you're concerned with having too much Co2 at night, have an air stone of some sort on a timer to come on when the lights go off, and have it turn off about an hour before the lights come on. I do this because, when the lights are off, the plants cannot use the Co2 so it can build up in the tank. On a pressurized system, you could turn the Co2 off at night. But since we use DIY, we can't really turn it off.