Question for U.S. real estate experts?

keep getting these notices from companies telling me I've overpaid my real estate taxes, and they'll recover them for me provided I sign an agreement stating they get to keep half of the refund. I went to my county real estate tax website and put in my information, and it said I haven't overpaid anything so I don't qualify for any refund. But I keep getting these letters from private companies that say I do, and they'll get it for me if I sign the 50-50 agreement. Is there any other way for me to make sure the county doesn't owe me anything? I doubt the companies would be sending me the letters if they didn't stand to gain something.