You could feel it. Your energy would feel dirty and strange. You would have bad luck, accidents, illnesses. Feelings of anxiety and depression. Really just be aware of negativity that would follow some sort of pattern. At least from an elementary perspective. I guess you would be able to actually see it in your aura, if you were advanced enough. From my experience, being "cursed" can be as simple as people randomly thinking shit about you. This directs negative energy towards you. As you grow your aura of protection, this is completely ineffective. I don't have experience with being cursed with black magic (I think), but you can program your aura to repel and deflect any of that crap.
It's best to do this daily, preferably several times a day, so that if someone happens to curse you, it will just backfire. And most of the time it doesn't even reach you.
https://www.joyofsatan.org/www.angelfire.com/empire/serpentis666/Returning_Curses.html