There are some non-natives who speak English with a proficiency that may very well exceed that of the average native speaker. Some have taken to calling them pseudo-native speakers. In most situations, it’s extremely difficult to tell them apart from “true” natives. That said, there are some details that could give them away.

The simplest indication could be their accents, which could borrow pronunciation from a number of different regions. In addition, they usually either refrain from using colloquialisms, catchphrases and slang, or use it too much. Importantly, they may also be unaware of some conceptual knowledge usually learned by children during their formative years. Most pseudo-native speakers are also very curt and businesslike in their communication, preferring to eschew small talk. This may appear rude to some. Finally, they have trouble being consistent in their language use and in their judgment of other people’s language use.

Source: Peter Medgyes, “2.3 Pseudo-native speakers”, The Non-Native Teacher