Pure saffron tastes slightly bitter. It does not taste sweet. If your saffron is tasting sweet, it may not be the one you want. Also, check for the ISO 3632-1:2011 certification on the pack.
You can also put a few threads of saffron on your tongue and suck them. Take them out and rub them on a tissue. If they color the tissue yellow, they are real. Otherwise, they are not.