How to get conclusive evidence about if a MacBook Pro video or logic board is malfunctioning?

"I think Apple rather underplayed it there - the error is in the manufacturing and simply, it is a matter of when and not if... so, if you have one of those models, it is likely to fail.

If you are seeing flickering, most likely it is affecting you right now.

the only way to really test it is just to play high definition video, games, rendering or anything that can max out your video card for a period of time - however as the problem is heat related, doing this will make it even worse, possibly to the point where it will not boot or be usable.

I would just go for a repair now if it is free."

Your computer came with an Apple Hardware Test DVD. Boot from it and run everything it has to offer.