Staining vs transparent watercolor paints
Usually you can't, as the packaging generally doesn't show this information.

What you can do, is


test the watercolour out in the store, which is often possible (if no direct means are given, you can ask an employee),
look up some reviews and see if they refer to the staining properties, or
look up or into the brand's colour charts:

Some brands or ranges offer colour charts which tell you the staining levels using triangular symbols similar to the square shapes of the transparency level, like in the lower right corner here:



source

Where △ = non-staining, ◭ = semi-staining, and ▲ = staining.

Others might use numbers to indicate the level, as is shown here:



The four symbols underneath every colour indicate 


lightfastness (using Roman numerals I - IV), 
the staining level (using Arabic numerals 1 - 4, going from no staining to the highest level, respectively), 
granulation (Y/N), and 
transparency level (⚪, ◑, ⚫).