How do I know if a website is illegal?

How can one determine if a website is operating illegally, especially in the realm of cryptocurrency and finance? Are there specific red flags to be wary of, such as a lack of transparency in their operations, or the absence of regulatory approvals? Should users conduct thorough background checks on the website's ownership and history? Furthermore, are there any reliable resources or tools available to help verify the legitimacy of a website before engaging in any transactions?