There are multiple addons that incorporate some kind of range measurer. For a stand-alone addon, Curse.com suggested RangeDisplay
It won't show you when Starfall will hit your target, but it'll show your approximate distance to it. To find out whether or not Starfall will hit, you'll need two numbers:
1) Starfall's range is 40 yards.
2) Starfall's range when glyphed is 20 yards.
From there, it's an easy 'if then' equation:
1) If my Starfall is not glyphed then I need to stay within 40 yards.
2) If my Starfall is glyphed then I need to stay within 20 yards.
A side-note on the first point is that an unglyphed Starfall has the same range as all your other spells, ergo, if you're in range to cast Wrath and Starfire, you're in range to cast Starfall. If glyphed you'll have to resort to either the range display addon, or hitting Starfall and then moving closer until stars appear above the boss' head.
As for Ozruk
in Stonecore Heroic, his Shatter
has a range of 15 yards which is displayed by a ring on the ground when he casts it, so as long as you stay just outside that ring, you'll hit him with Starfall and avoid being hit by Shatter.