Odd question: what does trailer sway feel like at first?

The early stages of sway feel a lot like laying on a bed on a cruise ship or riding on a train. It starts off as small gentle rocking motions. Similar to the little wiggles that you feel in windy conditions. In the early stages it feels awfully benign and could easily be confused with normal conditions. It can be pretty difficult to pick up on the early signs just from feel alone.

You'd probably be able to pick up on the signs visually first. Normally when the trailer wiggles it eventually gets back in line with the TV even if it's only briefly. When sway is occurring the trailer never stops it's back and forth motion. It just keeps going and going.

As the condition becomes worse you'll find that the wave of the rocking starts to become more spread out as the trailer begins to oscillate. You start to feel the desire to make small corrections with the steering wheel, but doing so just makes things worse. Eventually the rocking starts to feel a little more violent and your natural pucker factor starts to set in. Eventually you get to the point where you're just along for the ride.