The best approach to pursue is usually multidisciplinary and you should have a process in place also starting from product and process design and development.
Marketing is also involved because in the technical sheet of the product, which can be also available on a company web site, there is usually reported the indication "RoHS compliance yes".
Quality is also involved to verify that the ISO standards clause regarding this requirement are respected by means of the internal audit.
Engineering as well as Purchasing are other function that are deeply involved , because they use qualified raw material in the product realization process and Purchasing shall acquire material that are compliant with the regulatory and statutory laws.
So , we can say that the organization shall have a process in place to assure that its product is ok with RoHS, assuring the presence of allowed material with related percentage and eliminated banned material. This is an internal process that Marketing shall communicate to the customer with technical sheet. Hope this helps