How do I know if I'm living my life on the Slight Edge?
Well, besides doing the obvious, daily habits that you are already aware of as necessary for daily improvement in the different areas of your life...
I think being on the Slight Edge should literally feel like you are on the edge. On the edge of your fears, on the edge of your comfort zone, on the edge of your self-imposed limitations. I think 99% of the "battle" is realizing that the edge is a good place to be. The pull of what's comfortable and known wants us to think that the edge is bad, that it's foolish and risky to try things we've never tried before. But the edge is the best place to be. The edge is where things happen, growth happens, opportunities happen, epiphanies happen, your better future happens.
Living on the edge is a lesson in the art of learning how to be comfortable with being uncomfortable. This is a practice, a skill even, that will take you far. Remember the Slight Edge: "What's uncomfortable early becomes comfortable later."