Altruism is a very complicated thing because there are many aspects to it, but the definition of Altruism is: having unselfish concern for others and helping purely out of desire and not out of personal gain
Altruism happens because of many reasons, three of these reasons being; Biological, Neurological, and Environmental reasons. One of the reasons being displayed here between Shara and her oldest child:
I will be fine honey but thank you for the concern!
Mom! let me help you shouldn't try to lift heavy things you could fall and hurt yourself!
Altruism also happens because of Social norms and Cognitive reason, Social norms usually state that you should treat the elderly with respect and Cognitive reason makes you want to help a person who is emotionally distressed.
Are you okay mister? Do you need a hug?
One of the reasons ( Empathy ) is very important to learn at a young age so when you get older it is a natural thing in our lives already.
Are you okay? Do you want a hug or for my to get Mrs.Applegate?
I want my mommy...
You are more prone to helping somebody if you are close to them, the more relation you have to them the more likely you will help them with out trying to gain anything from it.
All in all there is no true way of knowing if something is selfless but altruism is needed in the world to make it a better place indefinitely.