how do you look up an email address?

Depends what you have to start with. If you know the person and company, most work emails follow the same pattern (first.last, first initial plus surname), so check how other staff at that company show theirs and try that. Their LinkedIn contact info, the company's about or team page, or an author bio sometimes have it outright. Before emailing a guess, run it through an email verifier so you don't bounce.