Easiest way to tell is if it's in the fantasy section. The title gives away it's fiction.
If you know how magick works you can figure out by the title of most spells. Magick is energy found in nature, if it promises sudden dramatic physical change, it doesn't work because magick cannot go against nature. It also creates subtle changes in the energy around you to bring your desired change.
The remaining spells should theoretically work, however, sometimes people add items that don't. Example, someone makes a love spell but includes ingredients with little to no 'love' energy to them [think poisonous plants] this spell would probably not work since the items are working against the spell.
There are also 'grey areas' which are debated like death spells and weather spells. On one hand it's still dealing with energies, on the other this seems like too much power for a human. While I don't recommend trying them out, you should think about it and draw your own conclusions.
Finally, remember even the best spell won't work 100% of the time, so don't automatically blame a spell for failing. Sometimes the person you cast on was stronger than the spell, you didn't charge enough energy, you let your mind wander while casting, or you really didn't need the spell. Whatever the reason, don't conclude the spell was a fake when there are other options.