It is a good question. I think sometimes feelings of self hate or loathing are so strong that it's clear. Sometimes patterns of behavior that are persistently self-defeating, or subject one to disrespect might indicate lack of self love. A persistent inability to trust others goodwill, or a persistent lack of self-esteem could indicate not having a healthy self-love. Lots of ways.

Subjectively, I suppose the feeling of self love might be a calmness, content confidence, a belief in being worthy of love, a sense of interdependence rather than dependence in relationships, self-respect, self-liking.