Use the athletic page on college websites to get the backgrounds of players on the roster and compare yourself to see if you'd fit.
Pick a typical D I, D II, and D III program. Go to your specific sport on the school's athletic page. Look at the players' size, weight, and speed. Suppose you're interested in college basketball recruiting. If you're 6'3" and playing a forward in high school, you'll probably find that forwards in college are at least 6'5" at the D III level and closer to 6'8" at the D I level. You may be too small for the level you want to play at, or you may need to consider switching positions to make it happen. Same is true for college football recruiting. 6'1" and 190 isn't going to work when the typical player at your position is 6'3" and 245.
Check times if you're in a timed sport like swimming or track. That's a pretty straight empirical measure that's hard for a coach responsible for college recruiting to argue with.
Read the bios of current college athletes to see what kinds of awards they got in high school. They will all be impressive and it will give you a good comparison point for where you fit in.
You can also get a good feel for whether they recruit locally, regionally, nationally, or internationally. And you can tell if they're recruiting primarily from public, private, or religious high schools, or if they get a fair number of recruits from junior colleges.
Do your research. It's all right there on the web and it's free.