The simplest way I can define being "too selfish" is being someone who takes more than you're willing to give. I am a pretty selfish person, but I make a concerted effort to maintain balanced relationships.
"Willing to give" and "giving" are different traits. I might be willing to give but would not give because I'm an asshole, or because I'm scared of public opinion, or because I'm angry at the time etc.The simplest way I can define being "too selfish" is being someone who takes more than you're willing to give.
That's fair. And, in that case we can default to the "actions speak louder than words" scenario. However, the reason behind why you do not give can still define your level of selfishness. There's a big difference between me being willing to help you on moving day but having to work that day vs. me being "willing" to help you but staying home to watch Netflix 'cause I'm in the middle of a season.