As you are responsible for using registered substances only, it is your interest to contractually ensure that your supplier guarantees the validity of their registration. Your first option is thus to ask directly your supplier.
If the supplier has not claimed the information as confidential you can search via the registrant name or the registration number from the Registered substances page.
The full list of public active and inactive registrants/suppliers can be found in the Brief profile.
The full list of public active and inactive registrants/suppliers/registration numbers can be found in the Fact sheet.