There's not a lot of difference between a thinned shot and one you strike well. But you can feel the difference because the shudder transfers into your hands.
A thin shot is caused by the ball being hit too cleanly off the ground. The club strikes a point around the equator rather than the base of the ball. This is generally because the golfer tries to 'help' the ball up into the air.
To stop thinning shots you must feel as though you are sending the ball forwards rather than upwards. Use the tips on this golf tuition video clip to help you understand how to avoid the thinned iron shot.