One option would be go to the state courthouse in your county and ask to see any civil lawsuits that have been filed where your husband is named as a defendant. Also check in the small claims court.
This method obviously wouldn't help you find out if your husband is being sued somewhere else, either in another state or in federal court, but most lawsuits are filed in state court.
Anyway, the above suggestion might lead to something, or it might lead to nothing. But doing nothing will certainly lead to learning nothing, before your husband actually gets served. And then, of course, he'll know who is suing him, because it will say so on the summons and complaint.
Not legal advice as I don't hold Colorado licensure. If you need legal advice, please consult a lawyer who holds Colorado licensure.
It's also possible that there's no way to find out. Typically state rules of civil procedure permit a plaintiff to commence a lawsuit by serving the defendant, and only thereafter filing the complaint with the court. If the plaintiff's lawyer has chosen this method of commencing a lawsuit--a less typical method, but still one which is sometimes used--then the only people who know the plaintiff's identity are the plaintiff, his lawyer, and the process server. In that event, if the process server won't say who, the only way to find out may be to accept service of the summons and complaint.