Here's a suggestion: (Just jumping in, haven't seen if this has been used yet):
You yourself can tell the difference, obviously, no matter if others can or not. So write down two or three sentences or phrases, of two to four words each, that best describes YOU. Your core values, your "fight-or-flight" instinct, the values that mean the most to you (such as honor, or truthfulness, or safety, or pleasure-seeking, or being well-off, or guarding your friends, etc.)
Then, write down two or three sentences or phrases, of two to four words each, that describe your character's core values, whether those are friendliness, or greed, or selflessness, or selfishness, etc.) Have them nearby, on your character sheet or such, so that you can refer to them often in-game. Anytime you're faced with a hard choice, such as: Do I jump in and rescue the wizard? Do I trust what this NPC is saying? Do I offer one of my rations to the dying man? Refer to these tenets, just for a second, to re-center yourself.
Remember, when we're acting on gut instinct, and when we're not thinking, we react as ourselves, rather than as someone else. Eventually, you'll be able to jump into your character's head and think, "what would LAUCIAN do?" with little hesitation.
I do the same thing myself. Whether I'm playing crusader or knave, I eventually start jumping in to save allies, start trying to plan ahead, I take the safer path, etc. But I try to check myself every chance I get to turn it back around. About a year ago, one of my tougher challenges in an RPG was playing a character with a 6 WIS, because I try not to be impulsive with my PC's. I just played him as very unintuitive and impulsive, and it took a lot to leap before I looked, but eventually I got into it.