Greetings,
As a former DAT and C-DAT, yes the loader can look through the open breach to see if it is clear. Of course, that means it is not loaded, nor does the loader have control over the elevation of the main gun.
A much better answer is to have the gunner look through the Aux Sight (the manual sight aligned with the main gun). Once it clears dirt, the gun is clear.
I am not familiar with the historical layout here - Stereoscopic sights are on the upper side of the turret (M60A1), laser sights (M60A3 to M1A2) are in a dog house on the top of the turret. IF, the WWII sights are the equal to a modern Aux Sight, (aligned with the main gun) then - if your gun sight is clear, the barrel should be clear.
Regards,