If a spell is promising levitation, transformation, super human powers or instant healing then you know it is fake.
The more difficult ones to tell apart are those that offer realisic results but the process is fake, this is simply a case of reading and learning what and what is possible.
Also writing your own spells is what most experienced practitioners do, it gives you better control over the items, and ensures that you are connected to the spell, is it probably the best option but it may take some time to get a hang of it.
narrow it down quickly if it's in the Trick or Fantasy section, [i would also avoid the Weather and Beauty sections until you have a better understanding of spells] it's a safe bet they don't work. second way is to research the items used in the spell to see if they work with the spell at hand, third way is to study what spells do/don't work. finally, use a bit of common sense, you may find a spell that sounds too good to be true, it probably is [like say this chant and you'll become pregnant, or wear this herb and you'll lose weight]
I would just like to point out that some of these 'fake' spells could have a placebo effect. Even if they aren't real spells there is a possibility they would work for some people, through over coincidence or belief in the spell.
Examples of this: if you do a spell to lose weight, even if it wasn't a real spell, if you believe in it enough you may motivate yourself to loose the weight.
Another example weather spells if you do a snow spell and it is scheduled to snow, that is just coincidence.
I do stand by everyone else about the fantasy and trick spells though these spells will not make you fly/ turn into a vampire or werewolf no matter how much you want them to.
The only spells that I found probably real was finding spirit animal,creating psi ball and maybe just maybe figuring out your aura since mine was a little Wumbo jumbo lol. I want to learn some illusion spells that work lol but some summoning spells next one I'm trying is to create a spirit animal.