You need more than fluency and lack of hesitation to pass as a native. In the UK, as soon as another native speaker opens their mouth, we learn something about them. Every syllable we utter tells a fellow native speaker something about the part of the country we grew up in, the type of school we went to, and any social and intellectual groups we feel we belong to. It can be very subtle. For example, we can often tell when someone is trying to disguise a regional accent to sound more 'posh', or when they are trying to adopt a 'streetwise' sound to appear more hip or cool. But that's enough of us.

When it comes to non-native speakers, I can always tell when English is not that person's first language. It's one of two things. Sometimes it's the lack of regional or social 'colour' that can indicate a non-native. Some people sound so 'neutral' that it's obvious that they didn't grow up speaking English, and then you start to notice the non-native features of their language. In other cases, there's a mixture of pronunciations that gives them away. For example, a non-native who's lived in England for years might start a sentence with a fairly neutral accent, suddenly lapse into a syllable or two of exaggerated South London vowels, and then finish the sentence in a neutral way. No native speaker would do that.

There are also certain dead giveaways when when it comes to Dutch accents - the 'u' and 'ur' sounds for example. But if you're determined to fool that girl in the pub, why don't you record yourself saying a few things, and post us the link here? We can give you a few pointers as to what's giving the game away. Either that or stick to pubs with loud music and stiff drinks.....

Part 1 of 2

Very interesting topic. Well, technically the answer is yes. I have met people who learned English as a second language without realising that that was the case. But, without fail they had all moved to the UK as children and lived in the country for a sustained period of time.

So, apart from those, no I've never met a non-native speaker who could convince me that they were from the UK. I've met loads of people who were perfectly fluent, had great vocabulary etc, but within three seconds of them opening their mouth you know they are not a native speaker. Being fluent is actually only one of several factors that you need to pass for a native. 

First of all, if you have any first language interference at all then that is going to give you away instantly. That is a given. Secondly, as Su.Ki mentioned, when you hear someone from the UK you can tell what part of the country they are from, what is their social background etc. That is going to be unbelievably difficult for a learner to replicate.

You can tell within a couple of minutes of talking to a fellow native speaker whether that person is from the north, the south or the middle of the country. With non-native speakers this lack of regional accent, even if they do not sound foreign, is actually a big give-away. As well, sometimes people pronounce words in a way characteristic of different English-speaking countries, or different regions within the same country. Again, a big give-away.

I did once meet a guy who spoke flawless English, but he had no regional accent at all. None whatsoever. Every native speaker from the UK will have an accent indicative of the area where they grew up but this guy had the most neutral accent ever. It was really quite strange. I later found out he was a native speaker of German, which I couldn't have guessed, but I knew for sure he was not a native speaker.

Once, when I was on a flight from Belfast to London, I was sitting beside a guy who started to speak to me.  He used some very common Northern Irish phrases and expressions that wouldn't be used outside my country and he had a perfect accent.  I didn't think anything about it until I asked him how long he was spending in London.  He said he was going to be there before going to his family in Paris.

I'm sure he knew by my face that I was flabbergasted!  I asked him if he was French and he said yes.  I asked him if one of his parents had come from Northern Ireland, but as it had transpired, he had come to Queen's University, Belfast to study for a PhD in Engineering and had basically perfected the accent.  During the whole conversation there wasn't a single word, phrase or expression that I could have faulted him on (I wasn't looking for faults, by the way!).  He had the accent, expressions and inflection in his voice absolutely to perfection.

And whilst I did wonder several times if he was joking, his French passport proved it beyond a doubt.

Since then, I've met several people who come very close to sounding like a native of some region of the English speaking world, but the conversation with that guy will always remain with me.

Part 2 of 2

Thirdly, there is how the language is used in informal and colloquial situations. Native speakers can be quite lazy in informal situations, using slang, not pronouncing words clearly and people who have learned to speak the language really well may actually be more grammatically correct and formal. Again, this tone can be unusual to native speakers. I think getting grips with the colloquial language as well as more formal language is essential if you ever want to pass for a native. But again, there-in lies the problem because it differs from region to region!

I think I know British accents too well to ever be fooled by someone. If I were going to be fooled it would be by someone with an accent from another English-speaking country. Perhaps you might be able to convince a native speaker of English, but not from the UK, that you were British one day. I think that is a more achievable, although undoubtedly still very difficult goal.

I could see that happening, because people are less familiar with accents outside their own country. I know it is possible to get a superb native-like accent as a learner. One example is the polyglot, Luca Lampariello who has in my view a pretty flawless American accent. There are a few minor sounds which he has difficulties with very occasionally, but to me it sounds outstanding given his first language is Italian, he has never lived in the US and doesn't have American family. But then I am not American. Probably an American could pick up on some minor faults better than me. If I am going to be overly critical, his pronunciation is almost a little too careful, like he has learned it, rather than the pronunciation developing organically like it does for a native speaker. So, indeed, not an easy thing to do at all with English! Great question!

I don't want to sound like a major troll, but I actually do that sometimes just for fun, specially in bars and places like that. It's like a self test: how long can I pretend to be a local?

I did once a british accent in the US, and several times in Argentina I tried to keep the local accent. I must confess though, sometimes it didn't worked :D