How to determine if i have cancer or a brain tumor?

Pathology: I interpret your question is how to differentiate between brain cancer and tumor. Tumor is a generic term that can involve low grade and high grade tumor. Brain cancer refers to the high grade or malignant tumor. How a tumor is graded is done by the pathologist. Thus the necessity for a biopsy. There are features/markers such as the nuclear appearance of the cells that help to categorize the tumor.

Very rare : Brain tumors are very rare, so it is unlikely you have one. The most common symptoms is unrelenting headaches, especially if they wake you up from sleeping. So your primary care doctor who can examine you and look in the back of your eyes to look for increased pressure. A ct of the head would be the next step.