How would you know whether a fossil is REAL?
First of all it's important to note that most fossils are found under the ground and need to be repaired to some extent. This is normal and expected. Also, some fossils are enhanced, such as polished, to create stunning aesthetics. All of these are still considered to be authentic fossils.
On the other hand, other specimen might be replicas/casts or totally fake. Some replicas might still cost money due to their scientific preciseness, but they are not actually real fossils, and should be stated so by the seller!
Companies which certify fossils might make use of expensive techniques such as CT scans and X-rays in order to date, classify and authenticate fossils. However, there are other more straightforward tips that one can use. 
When you buy a fossil you should be provided with dating information as well as the species and where it was found. Research this. Some date-species-place combinations do not make sense. 
Also, research about the market value. If a relatively rare specimen is being sold at a ridiculously low price, it is probably fake.  Also, if the seller is selling an amount of identical specimen, that would certainly be a sign of fabrication. Every fossil is unique.
Look at the fine details. These are quite hard to fake, such as serrations on megalodon teeth and blood grooves on bones.
If you had to lick a fossil, it should stick to your tongue, though you might not like the taste! Authentic fossils are quite heavy and feel like rock. If a specimen is light and makes a hollow sound when tapped, that would be suspicious. Also suspicious would be air bubbles, which might form if the specimen is fabricated. Also, if it is fabricated using a mould, it might show a thin line along its sides from where the mould might have been opened to remove the cast.
Finally, perfection. What real fossils should have is not perfection, but rather, the lack of it! Repaired hairline cracks on the specimen is a good sign.
Unfortunately fabrication of fossils is an industry that exists. It is important to learn these little tips as well as to only acquire your fossils from trusted, reputable sources who guarantee authenticity and offer a money-back guarantee. You deserve a real fossil