you need another actor 1/4 of the of the player's height and maybe 3/4 of the width, lets call them "feet" and you need to constrain them to your player's X and Y-1/4 of players height, then make it so if feet collide with the ground a true or false, lets call this one "can jump" becomes true, otherwise it's false. If your player isn't a fixed rotation this gets more complicated, but it can still be done
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http://gshelper.com/?p=215
If your player isn't a fixed rotation this gets more complicated, but it can still be done
hope that helps