1. Speaking perfect English, both fluently and without a heavy accent.
2. Talking about technical topics or other shibboleths with the same cultural affect an American would, as opposed to a foreigner. For example, many Indians will call a phone screen a "telephonic interview", and I've personally seen Americans react poorly to that.
I know lots of developers who are very well paid and very effective in their jobs, but they don't speak English as well as an American and would stumble at certain cultural nuances.
But I always notice how a native English speaker almost always speak perfect English. They pick the right word to use and many other small things that make their English superb.
Any decent engineer joining Facebook in 2012 and sticking there until now would earn 60k a month.