I'm seriously tired of arguing with people, so I'm going to stop quoting people. It ought to be obvious that roleplaying is a skill, and like any skill you can be skilled in it and perform well (naturally or through practice) or you can be unskilled in it and perform badly.
Signs you are roleplaying badly:
1) You are playing yourself, and that's not a conscious choice. Your character has all of your own biases, beliefs, and inclinations despite the vast differences between your own background and experiences and the one you gave the character.
2) You continually while playing the game address other players about in game matters (such as tactics, puzzles, past events, etc.) rather than have your character address their character. In particular, you do this without even realizing you are doing it, and have no problem saying something to Jim (another player) like, "Jim, have your character move around to the other side of the hobgoblin so we can flank him.", rather than on your turn saying something like, "Sir Didimus, circle to my foes rear so we may out flank him."
3) Rather than in engaging in dialogue with the other PC's or NPC's, your first instinct is always to describe what your character intends to accomplish. For example, you might say, "I go up to the Baron and introduce myself", as a proposition rather than something, "I go up to the Baron and extend my hand in greeting, "Good morrow, your Lairdship, I'm Sir Bannet of the Lantern Company". Or you might offer as a proposition, "I try to convince the chieftain that he should allow us safe passage through his tribes hunting grounds."
4) When imagining events, you typically imagine yourself looking down on your character or looking at your character do something, rather than imagining you yourself doing something, or even worse you don't imagine events at all but always think of them entirely in terms of game states or game mechanics (the same way you might think of a chess game).
5) Your character is anonymous. No one else in the party knows any details about your character such as his name, height, hair color, habits, preferences, or in some cases even race ('You are a half-elf, I never knew.') - all things their characters should know - , but all the other players know your class, strength score, level, and BAB - all things their characters could not know.
6) Your character is a complete blank slate when it comes to personality. Regardless of your characters stated beliefs and background, whatever the game situation you encounter, your characters actions are always the ones you as a player deem most expedient (or see #1 above) and particularly most expedient under the rules you are playing. Regardless of background, class, or stated beliefs, your character's alignment can be best summed up as 'neutral survivalist' or 'chaotic greedy' (yet you refuse to play this alignment). The corollary to this is your character's personality or stated personality (such as it is) is dictated by whatever personality you think offers the most mechanical rewards in the game.
7) You have virtually no direct interaction with the game world. All the propositions you offer to the game referee are in the form of rules propositions: "I attack.", "I make a search check.", "I move six squares.", "I attempt to turn the undead.", "I cast magic missile at the darkness.", etc. This is most easily seen by the fact that you never offer up any propositions that don't have a mechanical effect on the game, so for example, your character never eats breakfast, cleans his sword, takes a bath, takes off his boots, or even indeed touches anything in the game world except implicitly as a result of a rules proposition.
8) Every one of your characters is identical (see #1).
9) You've spent literally hours outside of the game figuring out how to optimize your character up to level #20, but you don't know any of the following: the name of your character's mom, where the character was born, how the character spent his childhood, what your character was doing the day before the adventure began, what the character is afraid of, etc. (And if asked about any of that stuff, you'd probably say it was irrelevant.)
10) Your character description and mental image is essentially that of a anime character and includes any or all of the following words and phrases: "darkly handsome", "brooding", "moves with cat-like grace", "piercing intelligent eyes", "dangerous", "long black cloak". I'd almost throw "tall" on that list, but concievably tall might describe a non-twink mental conception.