Most ATC units use both Primary and secondary radar.....lets assume you are recieving a RIS "Radar Information Service" the controller will advise you of traffic he see's, if its a primary contact he will still call it to you and probably advise it as such a contact.....if its a secondary contact with a mode "C" height readout will advise you of the contact and the height indicated but "unverified".
If he is using a secondary only radar ( i.e. here at Scottish using the Stornoway Radar) he will advise the service is limited to transponding aicraft only.