I have overdosed my fish with CO2 too many times to admit. Every time I did it, the tetras, and similar fish all clustered in a top corner of the tank, usually a corner where I would expect the CO2 concentration to be the lowest. Other fish would lay on the substrate, at times laying sideways, and some would have badly faded colors. To me those are absolute indications of too much CO2, with too little oxygen in the water to compensate.
When I first started using CO2 I noticed a lot of stress in the fish at what I soon learned was barely over 3 ppm of CO2. Eventually I realized that the only stress was what I was feeling from fear of harming the fish. Once I started using a drop checker I found that I was able to stifle my fear and increase the bubble rate enough to get significant CO2 in the water. For me that was the primary reason I used a drop checker, something I recommend to anyone who has limited experience with CO2.