Wayne,

The only way to check the compression is by removing the injector,
their are special tools for this. It's not a difficult job, but you do
have to reset the rack, which can be a little difficult if you've never
done it before and don't have the tools for it. By the way, two cycle
Detroit Diesel's have exhaust valves in the head (usually four valves)
and an intake port cut into the side of the cylinder (no intake
valves). The blower builds pressure in the intake manifold and when the
intake port is open that pressure, along with the open exhaust valves,
is what forces the exhause out.