For AAD-B2C it is fairly simple. From Azure Portal with a B2C directory associated, go to your B2C directory (I added the "Azure AD B2C" to my portal's left menu). In the B2C directory click on "User flows (policies) directory menu item. In the policies pane click on one of your policies you previously added to select it. It should open a pane for the policy. Click "Properties". In the next pane is a section, "Token compatibility settings" which has a property "Issuer". Your AAD-B2C tenant GUID is contained in the URL.