First of all, to demand moral action be consciously separated or refined in any way is simply prejudice against the stupid. If a human being is supposed to be capable of making as purely moral an action as an angel, with its much more highly developed intellect, if less consistently, then the stupidest human who still has any intelligence at all, should be able to be as moral as the most intelligent.
Therefore, it cannot be necessary to know or understand the reason for a moral act, in order for it to be moral.
The ultimate reason for acting, if it arises noumenally as noted by virmaior is still going to have phenomenal representations, and there is no reason why those representations cannot be sentiments. So the question is whether your sentiments are representations of duty, or whether they are completely tied to your own comfort.
When it works, I do believe this is a discernment that even someone with limited attention can make, since one of the moral criteria is the very concrete means-ends one, and we are to trust that the moral criteria clearly identify duty.
Even as quite young children we know when we are acting with respect for others, and when we are play-acting it. To the degree your motives are notably mixed, you are to some degree using people as means. [Being nice to your sister so mommy will be nice to you is not acting out of respect for your sister.]
Absolute discernment in cases of mixed motivation is not truly necessary because, given the absolute isolation of thought from deeper meaning in the noumenon/phenomenon split, it is not possible. It becomes a chicken-egg problem -- which came first, the motive or the awareness that you will benefit? I, at least, can almost never tell.
To my mind, this is consistent with a little bit of Calvinism that subtly inflects the Lutheran Pietism that Kant was raised with. To some degree, only God knows whether you are doing the right thing, but you get points for trying (even if all of those points have already been accounted before you were born, which kind of takes the payoff out of trying. But expecting a payoff is the wrong thing to do, anyway. And so on, back and forth endlessly.)
Analyzing motivation is not pointless, because that itself is a moral act motivated by the attempt to be clear on one's duty. But in the end, you can't attain access to pure duty by thinking about it. You might be able do discern that your action is correct, but your understanding of your motivation is always clouded.