how can you tell when cancer is arising?!

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You have hit on the crux of the problem Errin. If a cancer "arises" in a breast for example, it begins as a single cell which reproduces or "doubles" without the usual body control over cellular divisions. That one cell and its daughter cells double 30 times to reach approximately one billion cancer cells. One billion cancer cells weighs one thirty-third of an once and is about 1 cm in diameter, roughly one third of an inch. We are hard pressed to find a primary tumor in a breast this small even with screening mammograms. A tumor this small causes no symptoms, and yet it has divided three quarters of the number of times it needs to end the life of a person. If untreated, that small tumor mass will double 10 more times. After 40 total doublings it will reach an estimated total mass of one kilogram or 2.2 pounds which is often enough tumor mass to kill the host person. Along the way small parts of the original tumor mass - maybe 10 to 1000 to 1 million cells - may break off and travel in the bloodstream or the lymphatic vessels to other parts of the body such as bones, lungs. liver, brain, adrenal glands, etc., etc.. That is stage IV breast carcinoma which is no longer curable. We cannot see 1 million breast cancer cells anywhere in the body with any scan known to humans. It is too small to be seen.
So - we can only see relatively large amounts of cancer cells - tumor masses.
People so often fail to grasp the idea that cancers are microscopic diseases for the majority of their disease courses. Also, an amazing number of people think that cancer is one disease when it is truly a group of over 200 different types depending on which body cells go "haywire."
The course of an arising cancer of the lungs, colon, prostate. etc., etc., etc. is similar to the above simplistic description. I am trying to make this understandable for most people.

People should throw out the "Seven Warning Signs of Cancer" except for the change in a visible, palpable mole on the skin - the only useful sign. We must find cancers BEFORE they are large enough to cause signs or symptoms - hence screening tests advised for breast cancer, colon cancer,and prostate cancer

MD hematologist and medical oncologist - cancer and leukemia specialist physician for 20 years

Is the question how the person (or doctor) can tell when they have cancer and how it is diagnosed? Or how the process of cancer starts within the body - what starts it off, what happens to the cells, etc.

If you mean how does the person or doctor realize that cancer is present, it would depend on what type of cancer. Signs of colorectal cancer might be bleeding from the rectum, a change in bowel habits, pain with bowel movement.. which could also be signs of other conditions. If a person went to a doctor with these symptoms the doctor would do a colonoscopy which can detect if a tumor is present. A person with skin cancer might notice a new growth on the skin or change in an already existing mole, and this could be biopsied to determine if it's cancer. Some cancers aren't usually discovered until they are advanced, others are easier to detect in early stages.

I wish I knew. My Cancer was silent. If it weren't for my yearly mammogram I would have never known it was there. I felt no pain, and felt no lump.

Breast Cancer Survivor