The pictures, of the sight glasses, are actually showing fuel on the "supply side" of the fuel system.= being sucked into the transfer pump(lift pump).

We still use those "sight glasses" on the fuel return lines also. = it shows if air/combustion gases are leaving the engine's fuel supply "around the injectors". { Basically, you want at least two sets of sight glasses; one for incoming fuel supply=mounted right before the transfer pump; then one for outgoing fuel= fuel leaving the cylinder head.}

You need to insure you have good, solid fuel going INTO the engine, BEFORE you can judge the outgoing fuel in its sight glass(leaving the engine).