Well there are a couple of ways that you can find out! One way is to find out whether or not you filed "Form 2553 Election by a Small Business Corporation" to be treated as an S Corporation, and you have an IRS letter confirming that you would be treated as an S Corporation.
If you do not recall whether or not you filed this form, or do not have this letter from the IRS, then it is unlikely that you either you or your CPA filed for this election. Nevertheless, you should still try and find out your Corporatipon status and the best way is to call the IRS and find out. The IRS will tell you over the phone whether or not you are a Regular C Corporation or that you have a valid S Corporation election. You must provide them with the Corporation EIN#, current mailing address of the corporation and the officer name and details.