If the gun is horizontal with the thumblever up and you try to wiggle the barrels:
If the barrels won't move left and right then it is on face.
If the barrels won't move up and down then it is tight.
As you say, without the forend.
Mike, I'm not trying to be argumentative, only want to help the OP get the correct understanding he asked for, but your statement that I highlighted in red is not a given. I just now walked out to my shop and got out a Philly Fox A, which has a set of 32" barrels. I assembled the barrels to the receiver without the forend and did your horizontal test. Heck, I shook them every which way BUT loose. There is NO horizontal movement, or movement of any kind for that matter. I know what this feels like when it is there, as I own a couple doubles that ARE loose, but this one is dead tight. However, you can see daylight from top to bottom between the left barrel and the standing breech. It is tight however you shake it, but it is badly off face because of the visible gap I described. I had these barrels fitted to this receiver by a well known double gun smith, and he did a poor job. The off face condition is not from use, but is a result of poor work. Regardless of what caused it, it is what it is, off face.
Again, off face is when there is a lack of proper contact between the breech end(s) of the barrel and the standing breech. If it wiggles with the forend off, and especially with it on, it is loose in the action. Two different symptoms of wear or/and abuse, and not necessarily mutually inclusive of each other.
SRH
