How do I know if a Poll is legitimate or relevant to me?

Relevance

Survey outreach messages describe the intended peer group. If you’re outside that scope — for example, the message says "Senior PMs at B2B SaaS firms" and you’re a Junior PM in a non-tech company — you can simply ignore it.
If the Requester’s LinkedIn profile suggests they’re not a genuine peer (the "nosy neighbor" case), trust your instinct and ignore the request.