So it seems to me to come down to a test of limitations. A litmus test that can tell me when my thoughts come from egoic mind and when they come from my authentic self, my essence. And usually we can feel it in our body as well.
The concept that "when I am in limiting thoughts I might be in ego" could be used as an indicator, a barometer of the source of my consciousness. If I am in limiting thought, then I am in the egoic mind. When I am feeling free and unlimited thought, I am coming from my authentic self, my essence.
What are some of the limiting thoughts that I experience? They could have some of these emotions involved:
Therein lies the challenge when immersed in a "reasonable" world.