npubs that are part of Inkan identities are recorded on Ethereum when the identity is created.
The recording of an identity currently costs about $0.80 - $1.20 in gas fees. Users can make a request for sponsored payment of the gas fees at
https://www.inkan.cc.
I guess it's not a sure-fire way to distinguish npubs that have real persons behind them from those that don't, but it's definitely a signal.