Have you felt sick recently? Do you feel a 'heavy' energy around you most/all the time? Do you feel nervous/uneasy? Have you been having a lot of 'bad luck'?
Those are a couple common clues you've been cursed. Also, if you received an item around the time your luck changed or your enemy seems to have parallel good luck [you failed a test in a class you're good at while they pass.]
With that said don't worry, since 99% of curses are the result of you scarring yourself so badly you attract negative energy and curse yourself. First take a deep breath and don't panic. If you received a cursed object, place it in a box full of sea salt and burry it off property. [If you didn't get cursed via an object ignore step one] next cleanse yourself. Easiest is a bath with sea salt. As you pour the water over you visualize/feel black energy being washed away [visualize oil or tar] as the pure water replaces it. Pull the plug and visualize the negative energy being taken far away. Next cleanse your house [sage smudge or just sprinkle sea salt water everywhere] in both instances the key is beleief and visualization. Afterwards cast a protection spell on your house and wear a protection charm. Finally [most important] don't think about the curse. It's gone now, if you worry about it you're inviting it back. Curses/spells work when you believe in them, if you don't they will go away and not effect you.
If you want to prevent someone actually cursing you in future, look into a daily routine of grounding and shielding and a regular cleansing/protection [protection spells once a month and a cleansing every 6-12 months I've found work best]