How do you know if a lymph node e.G axilary is cancerous. Any symptoms?

It depends. This likely depends on the type of cancer, and/or if you have inflammation/infection. A cancerous lymph node in this region from, say breast cancer is hard, firn, somewhat immovable, and painless to touch. If there is inflammation or infection, it can be hot, swollen and sensitive/painful. If you have either of these, you should see your doctor for a full work-up as cancer can still be there.
See a doctor. A detailed history and examination is necessary to begin to evaluate the cause of enlarged lymph nodes. A lymph node in the axilla may be due to infection in the hand or arm, or more serious, possibly representing lymphoma, hodgkin lymphoma, or breast cancer. Please see your health care provider.