Beverages of any type generally contain some measure of water. However, in the case of some liquids, the non-water components take more water from the human body to filter out than the water contained in the beverage.
The most-known example of this is salt-water from the ocean - there is too much salt so the body tries to use water from the body to "water down" the salt water.
Given a beverage with unknown additives, how would one determine if the beverage would actually hydrate or dehydrate a person? If you knew the ingredients (and the amounts), could you tell that way?
comments seem to indicate that it depends on the solutes and tonicity of the solution: are there any other factors to be aware of? What (common) solutes will have the largest affect on whether the human body will be hydrated and why?
if you need "common" to be defined, consider it defined as: contained in many[more than 3] store-bought beverages