Jax-
IN YOUR JUDGEMENT was it a fair, fly, batted ball? That should be the only real guideline you need to determine an if it was an infield fly. From your post, it sounds like it was a (soft) line drive, but of course I wasn't there.
Remember- JUDGEMENT IS YOUR BEST FRIEND ON THE FIELD.
Don't worry about what is an "appreciable height off the ground." It is what ever you say it is!
If you MUST have a rule of thumb, try this: if you can't read the ball, look at your partner, look at the fielder (ordinary effort), and then say the words "INFIELD FLY-THE BATTER IS OUT!" before the ball is gloved or lands, then it's most likely not a fly ball.