From just casual observation, I think most serves are falling as they hit the back fence on recreational courts (mostly have 19-22' from the back line to the fence). It would be an interesting empirical question to see how hard the ball would need to be hit to have it rising on just leveling out when it hits the back fence (I realize court surface will play a role in this).
Here's a video of Verdasco practicing his serve (mostly 2nd) and most of the balls seem to be dropping when they hit the fence. https://www.youtube.com/watch?v=FmHeDYQ89zY
Here's a video of Sam Groth (the current world record holder for serve speed - if we believe the radar). https://www.youtube.com/watch?v=iC6BvMSpeOM
Groth's serves seem to be rising, but he's really hitting the ball. So, I'm going to have to agree with Lee that it should be rare for a ball to be rising when it hits the fence off of one bounce.
As to the original question, throwing ability might be the best general gauge of serving ability.