amrbar

As has been correctly posted, " see daylight between the barrels and the standing breech"

You also have to look for gaps between the water table of an action and
the barrels - ie where the proof marks are located and where they meet the
action. This can be indicative of twisted action as well as just being off
the face.

Remove the forend and wiggle the barrels?

Yes, that is one way - with actions such as Purdey and Holland
I take off the foreend and HALF open the action and then
wiggle the barrel - the reason is the action may look and feel
on the face if you don't do this.

Als0 - check that some "gunshop gunsmith" has not peened the lumps
with a hammer or squeezed the lump in a vice to "tighten" the gun.

500 Nitro