What to do if I randomly checked my public record and saw I was being sued by a collection agency?

Even though the debt was a marital debt, the creditor can still go after you for payment-- because they were not a party to your final decree and therefore are not bound by it.  If your ex- was ordered to pay the debt, you can file an answer in the creditor's suit, and interplede (a suit against your ex-), and require the court to order her to pay it.

You can also file a motion to enforce in your original divorce case since she has obviously violated the court's order by not paying the debt. 

The other issue that you have right now is that you mention this is set for a "pre-trial".  That implies they have served you with notice.  If you haven't been served, they cannot get a final judgment against you yet.  However, they may have convinced the court to let them do alternative service.  Go inspect the clerk's file to see how they claim you were served, if at all. 

For either case, it helps if you can have an attorney help you.  A consumer law or family law attorney should be able to help you with both issues since they both deal with collection and enforcement issues.  It's tempting to do nothing, but it's better to try to get ahead of it now before it causes more damage to your credit history.

You can contest the lawsuit without the retention of an attorney. However, it is wise to at least consult with one as to the legal process of filing an answer with affirmative defenses attached. The problem I see is that if the debt is a marital debt, you and your "ex" are most likely responsible for it. I suggest that you carefully read your presumed marital dissolution agreement and judgment to see who is actually responsible for paying this debt off.