How do you diagnose paraneoplastic syndromes?

Testing: In some patients, paraneoplastic syndromes are clinical findings that accompany existing cancers. These may include changes in the skin, bones, hormone levels, electrolytes, or organ function. Extensive blood testing or scans in a patient with suggestive findings can confirm the diagnosis.

No one way!: There are many paraneoplastic syndromes, with different symptoms and caused by different cancers. The diagnosis depends on the particular symptoms that a person has. These symptoms can be vague and have non-cancerous causes as well. See the article on wikipedia: https://en.wikipedia.org/wiki/Paraneoplastic_syndrome