The easiest way to see cast - in my opinion is with the gun upside down and looking over the toe. Does the toe, the trigger guard, and the bottom rib align or not?
Upside down,,,and looking from the muzzle works best at least for me.
Place the butt on the floor, hold the muzzle with one hand flip the gun upside down. Glance down the upside down bbl(s) from your standing position.
Any intentional or otherwise cast or toe-out/in is instantly visible as you look along the bbls and then onto the trigger guard and down the toe line of the stock..
It's like looking down a piece of lumber. It's either curved or it's straight.
If they say they can't see any,,a simple camera shot will certainly tell one way or the other.
