Vivace,
The only one that knows this for certain is God, naturally. no one else knows 100% what is in a person's mind but I would rather think of the act in a positive light[1] than follow what seems to be the norm which is see charity as done under some negative light.
Every single one of the Saints, most religious people, monks, priests, friars, even laity. I can say with 100% conviction[2] Padre Pio did all things purely out of the love of God as did every single one of the saints. We know this because of biographies written of them. Visit a monastery, a friary, a soup kitchen run by nuns. If you go to anyone one of these places I'm convinced you will human beings acting purely out of the love of God.
IMO one should refrain from '100% conviction'
[2]. It may seem trivially obvious to you and thus absolutely correct, but as you said, only God can know for certain, rather it is your own wishful thinking
[1] that causes you to conclude with 100% certainty that those people are entirely 'pure' in their actions. In the case of the soup kitchen, its entirely possible they are also doing it for themselves, such that they don't have to feel worried for the poor children. Not necessarily true at all but it is possible(it would be part of my own rationale, were I to run a soup kitchen), and thus to claim 100% is a bit much.
Edit: Oh, and hope you don't mind if I backtrack a bit? in defense of Dawkins, I don't think he said that its impossible for a God to exist. He does allow for the possibility that a God exists but maintains that he has not seen any compelling evidence for it. Hitchens as far as I've seen has been a bit more convinced but I do believe that he is open to new evidence, if any were to be found.