I'm surprised your doctor did not feel it! Just to be sure..is it a lymph node that you are feeling? They are located on each side of your neck, and become swollen if you are fighting off an infection or virus (just like the common cold, for example).
You most likely do not have a tumor if your doctor did not even notice your lump. Do you have a family history of tumors? If not, I would not "go there" in your mind and not jump to conclusions. Even if you do have a lump, most lumps are benign (non cancerous).
Yes, there are screening and diagnostic tests that can be performed to check for lumps. You would need to see your doctor, who would order the appropriate test for you, however. Do you have another doctor you can see for a second opinion?