Well, your total potential market is a cross section of:
- Racing Game Fans (with current PC racing games)
- iPad+iPod Owners
If I was you, I'd try and dig up some sales figures from some modern or recent racing games. Those might be hard to come by, but you might be able to scrape some data from the number of active TrackMania 2 players (or similar such online games). Actually, "active" players might be a more realistic market for your product than mere owners of racing games.
You also need to keep in mind you're competing in the racing wheel market, which may generally have better responsiveness and offer a better overall experience for the player (it's car racing after all, not hand waving). But for a couple dollars as an iOS app, you may still be able to drive a decent amount of impulse "lets give this a try" purchases.
As for ascertaining actual demand for something, you Google it. Look for people asking for the exact app you're asking for "let me play my PC racing games with my iPad as the controller". Now yes, you stand a very good chance of finding nothing, which may actually mean there is no demand. However, that's not to say nobody will want it once they know it exists. It could be something they don't yet know they want.