Simple every-day-item hydrometer for distinguishing regular soda and diet sodas
Better than a hydrometer, use an Abbe refractometer to measure the index of refraction, which varies with sugar concentration. It requires just a drop of liquid, and that small sample is unlikely to have bubbles, reducing density. You can make a refractometer to measure the sugar concentration, or you can buy one for ~$US20.

That said, diet sodas have been linked to a number of health issues, such as diabetes and diabetic retinopathy. You might be better off with unsweetened products such as seltzer, club soda, tonic water or two cents plain.

On the other hand, if you want to explore brewing or candy making, that refractometer could be useful...

À votre santé!