Well, that's why I use hip-to-waist ratio instead. Sources vary, but
here's a link. It is a lot easier to deal with overall percentages than absolute numbers.
I would use body fat calipers, but I have none and a little net research says if I want accuracy I'd have to be super specific about where I place them and would have to shell out some dough for "accurate" ones. However, I do have one of those scales that measures body fat through electrical resistance. Another way to measure was something like
this.
There is also something called the Body Mass Index, but I find it to be asinine. At 4'10", I should be something like 90lbs. Pssht -- yeah -- if I were 10, maybe. Just hitting puberty makes that unreasonable. It also doesn't take into account fat-vs-muscle, etc. A jock would be obese according to it. So, I'd put it in the same category as the scale: nice but not gospel for assessing health.
I would say, use the Hip-to-Waist Ratio or the Body Fat Percentage (or both, I guess) and also a checklist of everything you feel you want to be able to do. If you want to be able to, say, walk around your house or whatever without feeling winded, set up that as a goal and provide yourself with mini-goals, such as walking a quarter of the way unwinded, etc.
Edit: Using heart rate might be useful.
Here is a site regarding target heart rates. I know the chart starts off with a 20 year old, but if you're younger, it is just 220 minus your age is the max heart rate, and you should be going for 50% of that starting off and up to 85% of the max. While someone would most likely try to sell some sort of wrist device or something, you can also just check your pulse yourself or if you have a blood pressure machine, just take it before and after. It's not the greatest solution but it beats having to go buy a bunch of equipment. The fast way to check your pulse is either your wrist (if you drew a line from the middle of your thumb and traced down to the wrist, it'd be somewhere around there) or your neck (that jugular vein has to be good for something, LOL). Count it out for 15 seconds and multiply that by 4. If you happen to have irregular heart beats, you'd have to just wait the full minute.