Per the RoHS legislation, the CM is not responsible for ensuring compliance of the entire. This is the OEM's responsibility. You are only responsible for ensuring compliance of the solder/conductive adhesive, which can be done through cert sheets from these manufacturers or performing your own testing (if you are small, you’ll probably do the former).
However, it sounds like you are uncertain if your customers are expecting you to be an active participant in ensuring RoHS compliance. Therefore, I would strongly recommend that you communicate your concerns to your clients and, when appropriate, point out that Pb-containing parts are on the BOM.