The only sure way to know which channel does what is to experiment. Luckily you won't hurt anything by plugging it into the wrong channel. Just be sure to stand clear of the prop before turning anything on.

Different radio brands do things differently, and I may be totally off base here, but the same radio brand (e.g. Hitec) does two different things depending on which NATION you're in... Talk about confusing!

In the USA, Futaba transmitters have throttle on Channel 3, ailerons on 1, elevator on 2, rudder on 4. Shoot me if I got that messed up, but I am certain of throttle on 3. Rudder-only planes generally have the rudder plugged into the aileron channel, so it's more correctly termed "primary steering," IMHO.