Most fake products will have slight visual differences but the simplest way is to check for the CND serial codes which are etched onto every bottle in production. See photo below for the placing of the codes on both original UV polish(on the rear of the bottle) and New bottles(on the base).Batch codes match the code on the CND master boxes. If you purchase any CND Shellac that has no code they are fake and should not be used. Codes on some bottles particularly white base or white bottles can be difficult to see sop please check in good light !!  Authentic CND products(which are hypoallergenic and fully tested)cannot be used successfully along with counterfeit Shellac which may cause allergic reaction, damage to the nail plate and cuticles. 

Authentic Shellac Salons are listed here.....http://www.lovecnd.com/salon-finder/

If you see CND Shellac offered in retail boxes at low prices it is probably fake.Royal Mail charge £3 first class small packet rate for a boxed CND Shellac, only 90p for unboxed. As with most things in life, if it seems to good to be true then it probably isn’t !! 

For lots more info on CND Shellac please contact CND or visit their website...www.cnd.com.