Yarn would be great if you could observe it at speed, either from in the car or beside it, but I'm thinking it would go back to it's original position once you slowed down. If you used something that would stretch under pressure, but hold the stretched shape when you brought the car to a stop, you could observe it easier. That's why I was thinking duct tape, since it's got enough stretchiness to move a bit, but not break. You'd end up with golf ball like dimples where pressure pushed down, and domes where it pushed up. Then there's the problem of breaking up the boundary layer, but I think the pressures above and below the hood are great enough to overcome that.
Hinged flaps could probably be seen from the cockpit, and I'm theorizing you could tell the amount of pressure difference by how high the flap floated (or didn't). Again, just rednecking it out here.