Arm cancer how do you know you have it?

A mass. Usually it is painless. Report any mass to your physician; he/she will know what to do. Let me share a tale -- a new pathologist self-diagnosed a sarcoma and had his arm amputated. Afterwards, sharing his specimen, the other pathologists recognized a rare, benign mimic of cancer. Moral: a wise physician does not self-diagnose! best wishes.
See your doctor. If you are talking about a skin lesion or bump in the skin, i would consider seeing a dermatologist to determine whether a biopsy is warranted. The biopsy should be able to answer your question. To evaluate a deeper seated lesion you may need to see a surgeon for evaluation and biopsy.