Does the Pentacon Six TL have a light meter built in? If not, how does one measure light with it?
The viewfinder on this camera is interchangeable. The waist-level (top-down) finder does not have metering. There are also non-metering prism finders, and version with a light meter built-in — the "TTL metering prism". ("TTL" for through the lens, of course.)
If you did not have a built-in meter, you would either use an external light meter or judge exposure based on experience. (See also: How did people calculate exposure in the pre-light meter era?)