Very simple little tip. You can use spells like Blizzard, Farsight, ect. with the circle showing you the area of impact on an area you think their may be an invisable wall and see the green circle get stuck on the invisable wall before you bother walking into it.
This also works for some items, including the holiday item "Wand of Holiday Cheer"
Sorry if this was hard to follow... Headache xD