Well, the replica sunglasses will not be equipped with excellent manual work which can be felt easily by the touch of hands or the careful observation. You can put the sunglasses on the table to see whether it is in balance. Usually the fake one will not be in symmetric because of no delicate manual work.
Indeed, yes, there are a lot of fake sunglasses in the marketplace anyway. And usually, people have no idea whether their sunglasses are fake or not. However, there are ways to identify their authenticity. First up, you should carefully check their quality , then ask for their certificate and warranty which are quite important. Last but not least, try to speak to their after sales people to see if they are real. Hope your sunglasses are real.