You know a lot about lineage and tradition, and you know all the great families. When you meet someone, you can instantly tell that persons social standing, family and lineage. There is simply no confusing you on this; any attempt to use disguise and such where a person claims a different lineage than his actual one is automatically discovered by you.
You can instantly tell a person with BlueBlood on sight.
If somebody has been possessed or imitated by MonkeyMimic or similar body-stealing means, your ability may fail, but only rarely.