I suspect most just simply say "Yes" if they are able-bodied adults who think they can lift 50lbs (if they even process that much.) More likely most parties say "Yes" so they can stay in the exit row.

However, if in your mind you have the slightest shred of doubt, you should say "no."

I don't want someone who is paralyzed by fear and indecision in the face of an emergency blocking the window exit. Just as I don't like sitting in the window seats - I don't really want to wait for two people to make up their minds to evacuate.

People respond/react differently to such situations. If you're not someone whose knee-jerk reaction is to spring into action, please do not sit by the window exit.