Shy of owning some highly sophisticated, state-of-the-art drug testing equipment, you have no chance of knowing what you've got. After all, bath salts are not a group of drugs; the word is merely a euphemism, just like "plant food", "research chemical", or "herbal incense". If you brought it from a legit vendor ie not an illegal dealer, chances are it won't have any of the really juicy chems. (mephedrone for example). In future, I'd say you should buy the pure chemicals from trustworthy RC vendors; these "branded" bath salt formulations sold by headshops are grossly overpriced, as they are cut with useless fillers.