2)You find that out through trying it. That or reading the description of the skill, which would usually be detailed enough to let you know if it would be something really specific. Even if it's not so useful, it's a lesson learned for the next runthrough or respec, and still another thing you can do in combat. Besides, unless you're playing on a really hard difficulty, there probably won't be much reason to have to worry about a point misspent.
And people are complaining about having to play D3 more than once HAHAHAHAHAHA