It is very, very easy to see the difference between a native speaker and a learner. Yes, we both make mistakes, but we make different mistakes. The only time when I'd find it difficult to know whether a person is native or not is when he or she is speaking extremely well. Even when using slang, there are certain formats and collocations you need to use - yes, slang has its own grammar. It's far less chaotic than you think! The danger with learners using slang is that it exposes your learner errors even more... you'll sound less natural, not more natural. What kind of mistakes have you spotted? I'm curious to know.

The answer is no... and yes. Nobody over the age of around six really makes 'mistakes' in their own language. By that age, most native speakers have internalised the rules of their mother tongue and are, by and large, proficient language users. However, there are many millions of native English speakers who use examples of incorrect English in their daily lives, but these are not mistakes. For example, when a native English speaker says 'I ain't got no money', he or she is not making a linguistic error. This person is speaking according to the language rules of a particular social group. In a rough school in a poor area of an American or British city, where most of the kids say 'I ain't got no money', a kid who says 'I don't have any money' is likely to get bullied. He may say 'I ain't got no money' at home if that is how his parents speak, or he may modify his grammar according to the situation. Either way, he isn't making a mistake.

Yes. But to add a more satisfying explanation, there are variations of English among different communities and groups within which their loose interpretation of proper grammar is considered perfectly fine. A great example of that is the rule that there are no double negatives in English. As far as I know, this is a rule that is broken in just about every context outside of the highly educated.

There is a wide range between careful, formal written English and casual colloquial spoken English. However, much of what could be called "bad grammar" in the utterances of a native speaker is not mistakes in grammar, but almost a different set of rules... and a lot of shorthand, words left out, incomplete thoughts... and space fillers like "ums" and "ahs." Here is an example of a word-for-word literal transcript of spoken English by educated speakers. SHULTZ: But all, all of these things. It's just that, uh, it's the same, it's the same thing when we discuss steel imports or, uh, bunch of these other things -- shoes or what have you, and meat. And on the one hand, there is the, the groups that is pushing it; on the other hand there's the consumer. It, uh -- as much as -- HARDIN: Everybody have one of these dairy departmental committees studying something you can favor. SHULTZ: Well that's, well, Houthaker is particularly good at getting it, uh -- HARDIN: Yes. SHULTZ: [Laughter] These and, uh, I agree with making a speech or something. HARDIN: I don't care if he studies it if he can keep his trap shut. But, uh, if he posses us by, okay. Is this good English? Does it contain mistakes in grammar? I'm afraid that any grammatical errors made by native speakers sound completely different from the grammatical mistakes made by language learners.

To add to SuKi's excellent answer: Languages evolve through these "mistakes" or better said, variants in usage. Usage changes over time, and formal grammar lags behind the changes, so often what the "rules" say isn't the way people speak. Ask me to explain in grammatical terms why something is said a certain way and I may have trouble. Ask me what sounds right to a native speaker and I can respond immediately.