[Mobhack]

It's simple. Whichever observer you have selected becomes the one in control of the mission.

Therefore, just select an observer and see if it has LOS, if that is what you are worried about. If he cannot see the impact, try another if you have several. You don't even have to go to the bombardment screen the normal LOS display for units will work fine to check if LOS exists.

There is no such thing as a "wrong" observer, since any one will do. Barring the first call for a new fire mission perhaps, if one is skilled and so faster than another one.