If the soda burns the back of you throat, it's strong. If the soda does not burn the back of your throat, its weak. Simple.

Most kinds of soda are "saturated" with Carbonation (CO2). This is done at the plant just before the bottles are filled with the soda. When this occurs, the soda flows into the bottle under pressure and is capped before the carbonation can escape. Most kinds of soda can only hold so much carbonation in solution and as soon as you open the cap, the CO2 begins to escape, that is the noise you hear when you open the container and of course the fizz and bubbles are all CO2 coming out of solution. So, next how to measure this. The easiest way is to set up a CO2 trap. This is done by taking a glass 2 cup measure and filling it up with water and then inverting it in a big bowl 1/4 filled with water so that the water in the 2 cup measure can't escape. Then using a length of aquarium tubing make a special adapter. This adapter uses a bottle cap from a 1 liter bottle of soda you have already used up and put a hole into the top and put one end of the tube into the hole. Seal the area around the hole with clay or glue. Now you are ready. Place one end of the tube into the over turned 2 cup measure in the bowl and then take a new 1 liter bottle of soda and take the cap off and replace it with the modified cap. You need to do the very quickly so that you minimize the amount of CO2 you loose when you open the bottle. Then just let it sit. As the CO2 comes out of solution in the bottle it will push the water out of the 2 cup measure in the bowl. As this happens, you can see how much CO2 is in the bottle. To get all of the gas out, you will need to gently agitate the bottle every few minutes to begin with and then every so often through out the day. It will take 24 to 30 hours to completely degas.