The most important thing, as I've always understood it, is that it be on face even when the forend is off the gun. The forend iron, if fitted right and not excessively worn, will push the barrels tighter against the breech than they may be when the forend is detached from the gun. Thus, we check for off face, and looseness, with it off. With it off, if you grip the action in one hand, holding the barrels toward the ceiling/sky, and shake it lightly from side to side, you should feel no movement in the action. Then hold the gun horizontal with a strong light source behind it and look for daylight between the barrel breeches and the breech face of the action. Any daylight seen, or looseness felt when shaking, is a cause for concern. OTOH, if you feel no looseness in the action and can see no daylight at the breech at all, put your forend back on and go enjoy the gun. To further complicate the issue, a gun can be on face but have a bit of looseness when you shake it, but it'll be off face soon if you keep shooting it, you can be sure.
Be aware that a gun going off face, in the early stages, may be off face on one barrel only. I have seen doubles that were tight on face, with the top lever even a bit to the left of center, that felt loose when opened for loading. I don't like this, but it is not a cause for immediate concern, IMO. However, if the barrels are off face, even a tiny bit, it needs to be addressed before shooting it any more.
I hope that has not confused you further. I'm no Michael McIntosh when to comes to explaining the workings of a doublegun, I'm afraid.