I'm of the mind that genuine altruism doesn't exist for the most part. It's rare to find someone who routinely does good things for others purely out of concern for them and not because they are motivated by some "final reward".
But I look at it this way: if our "good" actions are not purely altruistic, then the best way to evaluate them is to look at the end results. If I do something good for others, but I am primarily motivated by my own "rewards", is what I am doing resulting in good for others? In other words, what is the net result? Even if I am acting selfishly, some selfish actions result in disaster and others result in "good". If my actions directly and consciously result in the improvement of someone else's situation, even if this is (for me) an added bonus to an action that is primarily selfish, I'm fine with that.
If I only do good because it makes me feel better about myself, that's fine if the end result is still helping others out. I think, IF karma does exist, then these sorts of actions should put me on the profitable side of the ledger.
However, I do believe (and I may be mistaken here) that part of karma and the rewards it offers are applicable to the next life. So if you believe in karma, then you must believe in some sort of reincarnation and consequentially, the possibility that you will NOT be rewarded in the current life. If you believe this and still do "good", then perhaps this is as close to genuine altruism that we'll ever get. For Christians, this applies to the concept of Heaven and Hell. But we will never know whether or not Heaven and Hell really do exist in the afterlife. But we still do "good" because we have FAITH that there is some greater reward.
To me, this is the essence of any faith: the belief in a greater reward (whether it be karma, Heaven, Enlightenment, etc, etc) and the pursuit of this greater reward through "good" actions despite not knowing for sure whether or not you will be rewarded at all. Faith is the belief in something without proof that it exists and without needing proof. Whenever I hear someone say that they KNOW that God exists or that they refuse to accept the possibility that He may not exist. I dismiss whatever it is they have to say about "Faith", because they simply don't have it. If you claim to KNOW that God exists, how is this Faith? You can't have "faith" that 2 plus 2 equals 4. Follow?