There is no specific way to know if you are being cursed by someone else.
1) Have you crossed a practitioner who not just could, but would curse you?
A curse comes most often as a malicious reaction to someone doing something against the practitioner. A curse requires time, effort and a willingness to "pay" (check the Rule of Three) on the part of the practitioner, and is not something an ethical person would undertake for "the fun of it".
2) Have you been told by someone that they have cursed you?
Not everyone who claims to be a practitioner is. In my experience most real practitioners would not undertake a curse. If you have been told about a curse by someone, I would be un-inclined to take them seriously as the Universe has a way of sorting things out. (once again, check the Rule of Three)
3) Curses tend to only work when you are willing to give them the power to work on you. A series of unfortunate things happening to you is not necessarily a curse, sometimes it is just a bit of bad luck. Things tend to go in cycles, so if you are currently undergoing a "nasty spell", things will soon sing 'round and you will be in a position to enjoy the "god stuff" too.