1st Make sure it is a realistic spell.
2nd The best spells are the ones you make/write yourself.
3rd Making sure you are well practiced
in your basics.
Make sure if your using stone, herbs etc that they are the right ones and that they are not going to harm you.
If calling anything to help with your spell make sure they are useful to you and that your not wasting their time.
Use common sense when it comes to some spells on this site. If it sounds fake or like it will not work then most likely it is fake.
To make sure a spell will work..First one must believe in themselves and magick. If you do not believe in what your doing it will not work. Also the best spells are the ones you write yourself. No one else knows exactly what you are trying to do except you.