A Whitepaper is only meant is to provide you an insight and better understanding about a project and how it works.

"A white paper is an authoritative report or guide that informs readers concisely about a complex issue and presents the issuing body's philosophy on the matter. It is meant to help readers understand an issue, solve a problem, or make a decision. - https://en.wikipedia.org/wiki/White_paper"

A Whitepaper is only relevant if the project isn't an outright scam to begin with, because any project could hire someone or firm to design its whitepaper and get a perfect one, So don't be surprised if a scam project has a very concise and detailed whitepaper. However, an experienced Investor might be able figure out if there are some discrepancies in the whitepaper.
Whitepaper might be one of the ways to find out but, cannot be solely relied upon to know if a project is legitimate or just a scam.