How does PCR show cell markers?

I'm an undergraduate working in a lab and am kind of learning and fending for myself, so i need some help. How does PCR show specific cell markers. I understand the pre-mix solution with the Taq, buffer, primers, molecular water, Q solution (enzyme) and dNTP then add your DNA template and put in the PCR machine but I don't understand how it knows we are looking for PECAM or others for example, help please!