Complex. Cancers, in general, are definitively diagnosed by biopsy (cutting out tissue and examining cells under a microscope). They also usually have distinct patterns of where they arise and spread. Hodgkin's disease, a cancer of lymph cells, may cause enlarged lymph nodes, with lumps in the neck, underarms, groin. Enlarged nodes, liver, or spleen may also be seen on chest x-rays and abdominal ct scans.
Biopsy. In order to make the diagnosis, you have to get a piece of tissue for the pathologist to make the diagnosis. There are many other types of lymphomas that require very different chemotherapies. Imaging and clinical picture can lead you to suspect hodgkin lymphoma, but obtaining tissue is mandatory. Also, there are many reasons for enlarged lymph nodes that are not cancer (infection, for example).