How can I know that I am fully concentrating?

I know I'm fully engrossed in a project when I lose track of time. It does take practice and a conscious decision to get my busy brain into that space (noise-cancelling headphones, putting away phone, muting email for a set time, etc.), but when it happens I always feel a sense of completion when I come out of that flow, even if wasn't able to finish my task. It can be exhausting, though, so it is best for me to have set time frames (30 mins or an hour) to work within or to chunk my work into specific tasks I can complete in less than an hour so I don't burn out too quickly over a work (or play!) day.

I know when I am really concentrating because I am not thinking of anything besides the task at hand! When fully concentrating, I can solve the problem at hand in an efficient and calm manner. I am not worried or concerned with other things!