Can someone really cast a spell on you and would you know if they did?

Yes someone absolutely can cast a spell on you. Whether you would know about it or not is another matter.

If you are experienced in witchcraft and spell casting you may be able to recognise the signs of work being done against you. Anyone proficient in the craft however, will have a decent protection system in place, meaning the spell will be unlikely to cause any serious effect. Some even go so far as to set up shielding, meaning the magick bounces right back to source.