It is difficult to gauge spiritual progress. We are too close to judge it for ourselves, and finding someone to do it for us is not always possible or desirable. Yet it is helpful to know how far we have come in our practice so we know what to do next.
Fortunately our ancestors have been working with this problem for a very long time. They would ask themselves questions or consider ideas they could share. Over time, conversations, reflections and ideas would come. For example, one idea they spoke about is a commonly accepted spiritual milestone. It is learning the difference between loneliness and solitude. Loneliness is being apart. Loneliness is being alone, it can feel like we are rejected, or shunned. Solitude though, is also being alone, yet it is about being profoundly connected, even though we are separate. Solitude helps us feel the connection with nature, or sprit, or history.

When we feel this difference, between being alone/lonely and feeling solitude and connected, it changes how we approach our aloneness. Being alone does not mean we are automatically lonely. Being alone can mean we are connected to some of the deepest awareness we can imagine. I remember when my father died. Even though I was with many people, I was alone. I was alone because I wanted to remember him, and make peace with our difficult relationship.

When we are alone, and we enter solitude, we have access to a deeper part of ourselves. This deeper part of our awareness, or our thoughts, has the ability to connect us to beauty more deeply. It gives us the ability to remember more clearly. It frees up our breathing, opening up our lungs. Finally, it gently reminds us that we are creative, loving beings.
Mary Oliver explains it perfectly.

“Make of yourself a light," said the Buddha, before he died. I think of this every morning as the east begins to tear off its many clouds of darkness.... The light burns upward, it thickens and settles over the fields... Even before the sun itself hangs, disattached, in the blue air, I am touched everywhere by its ocean of yellow waves... And then I feel the sun itself as it blazes over the hills, like a million flowers on fire— clearly I'm not needed, yet I feel myself turning into something of inexplicable value...