One of the Enhanced Features (see the rest here) is Lead Verification.

We have 3rd party data providers that pull profile data associated with a new lead's email and phone number. If the profile data matches across the various data profiles then we have high confidence that the lead has provided accurate info to you.

Verified Leads have a blue outline around their avatar in the My Leads table. If you open the lead's details, you'll see a "Verified" badge below their photo too.

Leads that aren't verified aren't necessarily fake though. Just means that we're less confident in their data.