How to ensure a RF module is indeed FCC compliant as advertised?
Buy direct from the manufacturer.

Likely ok: PTR9018 directly from Xuntong Technology

Unknown: PTR9018 from reseller

For larger deals, you may want to study up on "miàn zi", and develop some relationships in person (ie: travel to Shenzhen).  It is useful to have friends/acquaintances who already do business there, who can introduce you and/or act as your representative.

Most assuredly do not suggest that you want to avoid being cheated!  Ask for documentation with your purchase which includes certificate of FCC compliance, and give a non-offensive reason such as "because it is required by government agency in USA" (it may well be, for example for medical devices).