How can you be sure someone is casting on you?It is hard to tell for sure if a spell has been cast on you. It can be very difficult to determine whether someone is under a spell or not, because there is an infinite variety of spells in existence and each witch has his/her own techniques for spellcasting.
Some spells may bring about sudden, unexpected and unexplainable changes in someone's life. There are some spells that cause easily noticeable effects like falling in love with a someone you did not even like, having a series of incredibly good or bad lucks, having unusual dreams or visions, and the when hexed having odd mood changes during the day. Of course, if the hypothetical spell appears to be negative and causes you harm or goes against your free will you firstly need to reverse the spell and banish all of the negative energies sent to you. You may want to learn about ways to banish spells and to get rid of negative energies.
Some people get to the conclusion a spells has been cast upon them by divination. There are hundreds of ways to practice divination; you may use a tarot reading, a pendulum, a ouij board, a scrying mirror, a crystal ball, fire scrying, runes, etc.
You can do a meditation to find out if there is a spell on you. Focus on yourself. If you feel something off, something wrong, or something that doesn't feel like it belongs, then that is probably a negative influence or a curse. The best way to get rid of it is to think of it as a string or rope surrounding you. The string will have an origin, or a direction in which it came from. Sever the string and make it snap back to its owner. It will be like a rubber band; once it gets to its caster, the spell results will not disturb you any longer. I would also like to add that when you are experienced with magick enough, you can feel that a spell was cast on you by feeling the negative energy. We basically start with energy sensing and manipulating, so when we are into the craft for along enough we develop our energetic sensing skill and by using this knowledge we can assure if there is negative energy surrounding us or not. Some people might also refer it to empathic skills. It is not too easy for me to explain this since this is a feeling we get and not a fact that we learn.