A spell can become permanant, a well crafted spell has a release mechanism built into it. Spells do sometimes fade on their own so new solutions should be found over time, it depends on how strong it was as to if it will collapse.
A person who is sensitive to such things can detect a spell, especially while doing energy work. There are also spells to discover if you are spelled. Divination sometimes works, another way is if you have a familiar spirit or guide to assist you.
If you become good at shielding, especially mirror type energy work you can deflect spells.
Even better is not to hang around people nasty enough to put a spell on you, generally Wiccans and moral pagans will never cast spells on people without their permission if they fool with magic at all.