Firstly, you can judge by the price. Replica sunglasses are usually cheap, so the material is always cheap and the lens wouldn't be safe. While authentic sunglasses have high quality, so they are usually expensive.Secondly, replica sunglasses will not have the brand names on them while authentic sunglasses have.
Replicas wouldn't cost you much and some of them are in good quality and the lenses on them are safe to wear too. The only difference is the designs are not original but the copy from Authentics. For their cheap price, you can have several back-up pair for different occasions or to go with your different dresses.
If you buy a real one and replica one at the same time,you can distinguish them at once.Authentic sunglasses are much comfortable to wear,and the quality is much better.Once you buy your first pair of real sunglasses you will never buy a replica again.