If you have purchased through Ebay or Amazon then you can always check the resolution options in the video settings and see what the top resolution option is. (e.g. 2688 x 1520 multiplied together is 4,085,760 or 4 Million pixels which is 4MP)
But our personal opinion as an authorised Hikvision reseller is not to purchase from unauthorised sellers like Amazon or Ebay in the first place, not just because of potentially the wrong resolution but also the cameras can often be grey Chinese imports which do not support firmware from your region and in extreme cases can have Chinese language software installed.