There's no way to see how far away you can be from minions without missing the XP. When you hover over your XP, it should display a circle around your champ that represents the area minions have to be in for you to get XP from their deaths.
I think you can tick one of the options to show the experience "dropped" from minions (or is it a flying number on top of you to say you "gained" experience? Forgot) when they die, if you're within range to get it.
Have that ticked (I think it's off by default, possibly to avoid screen clusters, but nonetheless there if you want it), and you can work out the range rather instinctively.
