It is usually stated in the certificate, together with a guarantee seal. You can have the cut, carat, and price checked on another jewelry store if you want to make sure
The best way to identify or get a grading on a diamond if you don't have the certificate is to take it to a local jeweler. They will be able to either give you an evaluation or put you in contact...
You might be able to pry the stone out of the setting. A better idea would be to take the ring to a jeweler and ask the jeweler to remove the stone in a way that will best preserve the setting.