How do I know I have enough room to back down driveway?

It looks like you would have plenty of room to maneuver, as long as there are no cars parked on the street or anything else to interfere and make things a little more tricky.

From the first sentence in your post, I'm guessing this is your first RV. As others have said on this forum, finding a big empty parking lot to practice in before tackling your driveway would be a really good idea.

Once you get the feel for things, you'll feel much more comfortable dealing with tight spaces.