How can I figure out what "units" a SDL_MouseWheelEvent is in?

I have a MacBook Pro and the trackpad on it seems to give me a delta in “pixels”. This is great and I can implement per-pixel scrolling and everything works beautifully. Now somebody with a regular old click wheel mouse comes along and their delta is just 1. Now they have to scroll for an eternity.

There doesn’t seem to be a “units” field on the SDL_MouseWheelEvent for me to know how in the world I am supposed to handle these 2 different devices. Any ideas? Would adding this field be a good enhancement?