"So I believed clients when they said they did not receive a 1099-S"

There are five questions they need to answer Yes on, to avoid a 1099-S being issued. If you can get that, you can confirm if they were subject to 1099-S. Here's a version, although this uses True/False. I've seen item 2 be confusing under Y/N as a double-negative. It's the section for Seller Assurances:

http://www.titleguarantee.com/LegalForms%5CNJ%5CDeeds%5CPDF%5C1099_SForm.pdf