Magic Kingdom:
Except for Extra Magic Hours days (in which you should only spend the evening here), do not come to Magic Kingdom on weekends or holidays. It is excessively crowded on these days. Out of all the theme parks, this one will most likely be the most crowded on any given day (unless another park has a special event going on that day).
Look to two rides to decide if you should head to another theme park (most likely Epcot) or find something else to do that day:
Peter Pan: This ride is always crowded. If it's 55 minutes, you're looking at a pretty normal day. If it's more than two hours, you have an excessive crowd. If it's 30 minutes, you're looking at a nice day for riding. If its five minutes, then you should be able to ride every ride in the park within a few hours, because that is an excessively low day.
Tomorrowland People Mover: If you have to wait any more than 10 minutes for this one, head to Epcot, because you're looking at some tough crowds here.
Epcot
This park is a pretty easy park to deal with, even in a crowded situation. Even if it is crowded, you can still do the countries. However, if you're looking to do all rides, look to these two. This park is great for any day of the week. Holidays and during the Food and Wine Festival and Flower and Garden Show tend to induce heavier crowds.
Soarin': If it's about an hour to an hour and a half, you're looking at a pretty normal crowd. If it's 30 minutes, then its a light crowd (and by the way, if it is thirty minutes, then you should ride it. Like, right then, because you rarely see it). If it gets down to ten minutes, then the park is not crowded at all (and you should ride this one, because I've never seen it that low, and if I were you, I'd get on it). If its up at 120 minutes, make sure that Test Track isn't closed. If it is, that's why. If it isn't, then if Test Track's wait is about the same (which is pretty common) then you're looking at a heavy day.
The Seas with Nemo: If you have to wait more than 20 minutes for it, the crowd is heavy that day (unless everything else in the park happens to be broken down at that particular moment)
Hollywood Studios
This park is better done during the middle of the week, unless you want to see Fantasmic and it's only playing on the weekend. Star Wars weekends and holidays induce heavier crowds.
Toy Story Mania: Don't worry, even if this one is 120 minutes, you may still have a lighter crowd. This one tends to be crowded. However, if it gets down to 45 minutes, or god forbid 20 minutes (which I have NEVER seen, even at the end of the night) then you're looking at a light crowd.
Rock n' Roller Coaster and Tower of Terror: Check these as a combo. If both are up over two hours, then you've got a heavy crowd. If both are about an hour, your crowd is normal. If one is about 30 minutes and the other one is more, your crowd is normal to low. If both are 30 minutes, your crowd is very low. If both are under 30 minutes, then you have an extremely light crowd.
Muppets: If you have to wait at all for this one (other than just the preshow) then you have a heavy crowd
Animal Kingdom
This park is a great one for weekends because it generally isn't as crowded. For some reason, this one is more crowded middle of the week.
Expedition Everest: If this one is about an hour to an hour and a half, you're looking at a normal day. If this one is less than an hour, you've got a low crowd day. If it's five minutes, then you're either extremely lucky at that point or the crowd is exceptionally low. If it's up over two hours, then you've got a crowded day.
Primeval Whirl: This ride on a normal day will run about 30 minutes. 60 minutes is a heavier crowd, more than that is extremely heavy. If it's down at about 5 minutes, you have a light crowd.
I hope this help with determining the crowd. This isn't the only way to determine the crowd levels, but it's pretty simple.
The new queue area for Space Mountain
