Actually i can't find it. Maybe it's in feedback. B ut i remember posting about it. 
But you're right you should see the numbers. Maybe over their heads, or I'd probably prefer the number the pop up over your gold counter at the bottom.
And you just have to be in the area of the kill. No creeps stealing your kills, or having to compete with your allies over kills. 
I think there might be some bonus for last hit on either gold or xp, i'm not entirely sure, but i don't think it's a huge difference like in DotA.
I always thought competing with your own allies over getting the last hit was very silly.
