What is the best way to diagnose hiv?

I think your question needs careful consideration. It depended on the possible exposure. When was the possible exposure?

Testing is the only recognized way to diagnose HIV with specific intervals recommended from possible contamination time in order to rule out possible false negative tests.

Laboratory tests using blood and oral fluid can diagnose HIV. Visit this site for information on this topic :https://www.cdc.gov/hiv/testing/index.html Get HPV, Flu and Covid vaccines. Wish you good health!