There are body language cues that show you are thinking. Try looking up (like you're looking into your brain).
I also find it easier to do something with my eyes than to do nothing while thinking. That’s probably just me.
In the future, I might want an led embedded in one of my temples, that will blink like a network switch port or hdd light that indicates brain activity.
After a while it started to grate on me because it came across as “here comes my poorly formed first thought”