Quick newbie questions...

[ComradeP]

If it's revealed through air recon or recon spotting, it could be anything.

If you didn't recon the area (and it was thus spotted by the counterbattery fire mechanic after it fired), it's an artillery unit.

I'm not sure for how long the game has been going and if the AI moved the unit, otherwise you could load the scenario without FOW and see what unit or stack starts in that hex.

It can be difficult to determine whether it's worth firing on an unspotted artillery unit, as if it's likely to be a light or medium artillery unit you're probably better off firing at the enemy infantry. It depends on the situation. If you're worried about being assaulted in a particular sector, firing at enemy infantry units to try and disrupt them might be better. If you can spare the artillery, by all means fire at the unspotted unit.

Even though you can't spot the precise damage done in the case of an unspotted unit, it's still registered in the victory dialogue, so you can keep track of the losses (if any) that you inflict.