The simple test is that without blowing the shankh if you hear pleasant sound when you bring it near to your ear then it is a genuine one.
X-ray study can identify whether a conch is original or not. As there are lot of fakes in the market, the best way to test the authenticity of a shankha is by getting an X-Ray of it. The photograph is of an X-Ray. The X-Ray shows even the minor dents and spots of the shankha.
One of the main feature of genuine shankha is the rib/ridge like horizontal raised lines, at the inside opening. These ridges in the conch cavity, called Pilae, go through the inside of the shankha center, called columella, right to the top. People can create fake ridges on the outside of the opening by carefully filing away a part of it and leaving 3 or 4 faint raised lines to make it look like a real Dakshinavarti, but one thing no one can fake is this inside spiraling of the ridges to the top. This is seen clearly in the X-Ray.