Most likely ourselves because we are then able to "feel good" that we are so "long suffering" and "patient" and "self-sacrificing," because we can certainly see, from the evidence of our eyes, that the other person isn’t making any progress by virtue of our efforts. And, it may be a far more difficult thing to deny assistance, particularly when it is someone we love, because it "hurts us to see them hurt." Yet, that may be the very thing needed in order for them to grow - to be left to their own suffering until they have had enough so that they will begin to see their own way out of the difficulty, thereby building soul strength and accessing their own powers and inner potentials.
An STS vehicle does not learn to be an STO candidate by determining the needs of another. If one "gives" where there is no request, therefore no need, this is a free will violation!
If you give a child candy every time they ask for it, because you "love" them, or seek to "serve them" by meeting their every request, are you truly "serving" them? Even if their constant diet of candy leads to their ultimate physical death from diabetes?
If you intercede for your child every time he misbehaves in life, and prevent his experience of the consequences, how will he grow up?
Another aspect that needs to be understood is this: In 3rd density, we are ALL Serving Self in one way or another. We cannot, by our very nature of existence in the flesh that must consume to survive, be pure STO beings. That’s the bottom line. And, it is in the understanding of this, the acceptance of it and then focusing on learning the lessons of this estate in which we find our being, that we have the chance of becoming "STO candidates."
Generally we could say that respecting others' free will is STO
A checklist is hard to formulate, we could still think of some criteria:
Is the action requested?
Does the action presuppose a right to control?
Does the action open possibilities or create connections, internal or external?
Is the action self-contradictory?
Is the action covert and manipulative?
Is the action open-ended?
Is there a possibility of a positive reinforcement loop?