Borax, magnesium sulfate, & copper sulfate are among the most common -- but the container will not usually identify them, and certainly not by those exact names.

If you simply avoid buying diluted dichlor / trichlor, you won't need to try to avoid specific ingredients.

Regardless, that's your only practical option, since it usually takes a fair amount of knowledgeable 'sleuthing' to work out what's been added . . . and even then, some guess work is involved. You can learn how to tell WHETHER a chlorine product has been diluted, but probably not WHAT it's been diluted with.