The difference between ‘leading someone on’ and just being in a relationship is entirely internal, based on the internal states of each person. Partner A is ‘leading on’ Partner B if they present signals to Partner B to suggest they have intentions of a more serious relationship while actually having the intention of forming only a lesser, or even antagonistic relationship. If any part of that is not true, they are not ‘leading them on’ though there are other, similar ways for it to be unhealthy. As for signs, it depends on the ability of each partner to deceive Partner B. If Partner A is good at lying, rationalizing, etc. there may be no signs from them. And if Partner B is good at self-deception, they might fool themselves without A having to deceive them at all.