How can you tell if leg is broken??

Just now my cat jumped out of my arms and landed oddly. She's now limping very badly and when she sits she holds her paw up kind of awkwardly. She's very agitated. I was holding her a minute ago and she let me touch her leg, I was being very gentle but trying to tell if it was broken. A minute later she's laying on my bed and I'm just trying to look at her leg, not even touching her at all, and she swatted my face with her other paw. She's hesitant to step on it but she's walking around. She even kinda batted at one of her toys with the injured paw. But when she used the litterbox she scratched at the litter once, did her stuff, and left it uncovered. She usually scratches around for 10-15 minutes and almost never leaves it uncovered. She only jumped from maybe 3.5 feet off the ground but landed funny, and she's 10.
My parents want me to wait until tomorrow to bring her to the vet, but I'll be in class all day so I won't be able to watch her or take her in until 6. Is it likely her leg is broken or could it just be a sprain, and will she be okay until tomorrow evening? I moved her food and litterbox closer to her bed so she won't have to go as far but I'm very worried.

Only an x-Ray can rule out a break/fracture. Is there a small space you can keep her confined until you can get her to the vet tomorrow? Ideally you want her to keep as quite and inactive as possible in case there is a break. Certainly wouldn't want her to jump and miss and further injure the leg.

So long as there is no major swelling, the paw is still warm to the touch and there is no evidence of an opened wound, she'll likely be ok until you can get her in. Just get her in as soon as you can.

We live in a dorm room, so she doesn't have much room to move anyway, and she never jumps on anything, she's literally never jumped more than two feet except out of my arms. I have a carrier, but she hates it and I don't want to stress her out too much. Plus, I'd be out long enough that she'd have to use the litterbox. But she's so reluctant to get down off my bed- even though it's two one-foot steps down. For some reason she still climbs into my bed instead of lying in hers which is on the floor, but I wonder if that's because my bed is softer and thus maybe less painful for her leg.
Shes licking her paw on her injured leg right now, and her toes aren't stretched out so she can clean between them.

Your cat really does need an x-ray asap. Can you imagine if you broke your leg and weren't taken to see a doctor for over 24 hours?  

Cats are incredibly good at hiding pain.  There is just no way to know for sure but based on your description of her behavior I'd say she has a sprain at the least. Not many cats like being in their carriers but good cat guardians know at times it has to be done for their healthy and safety.  Better she should be annoyed for a short period of time than to have a bone which heals poorly if that is the case.  I understand your difficulty getting to the vet but please, do so sooner than later.

I wanted to go last night but I'm in a difficult situation. Incidentally, I sprained an ankle badly and my mother refused to take me to the ER for three days, and when I broke a bone it was four. So yeah, I can imagine.

She's still limping this morning but not as badly, and she was scratching at her litter like normal this morning. I'm reluctant to put her in her carrier because it is large enough for her to pace around and she doesn't just get annoyed. She will paw and scratch at the sides of the carrier, and I imagine however much pain she's in will not stop her. She will also resist going in enough that her leg would be in danger again. I'd rather she sit on my bed all day and limp to her box or food once or twice than shove her into her carrier and risk injuring her more. I know how good cats are at hiding pain, and I'm very worried about her. Due to the nature of my classes I cannot skip them, or I would be on my way to the vet right now.

My roommate (who's a veterinary science major and always had at least one cat around the house growing up) thinks that she wouldn't be walking if her leg was broken, she would stay put as a means of hiding her pain.

In the meantime, I have lots of first aide training and I have thought about splinting her leg; I don't want to injure her more by doing it wrong. I also want to know if she could appear completely normal 18-20 hrs after the accident (depending on the severity of her injury) because everyone here and my parents think she may recover by tonight, but I am reluctant to leave her without a vet visit.

There is no visible swelling and no open wound. From what I could feel she has a hairline fracture at most (I know those can get a lot more severe if left untreated). She really isn't a very active cat and I doubt she'd do much of anything while I'm in class. I've reduced the amount of walking necessary greatly by moving her food and litterbox as much as possible.

Yes, it would be possible for her to "appear normal" but still have something wrong. I can oftentimes take a severe break for a cat to withhold using the limb.

We had a kitten come into our rescue that was still using the limb (he was limping, but baring weight) and he had s pretty serious 2 bone break with displacement of the bone. Poor baby had been injured and the owners at the time thought he would heal on his own. He walked on the leg for over a week before coming to us.

I still think a vet visit is in order. Even if it is "just a sprain" they can give you meds to help with inflammation/pain. But, you simply cannot definitively know if it is broken or not without an x-Ray.

Get her in as soon as you absolutely can.

Thanks for the input everyone. The vet says it is probably just a sprain, because she is moving it normally, but it is a little swollen. I've got an appointment for X-rays at 11 tomorrow morning. In the meantime her things are still close together and I'm keeping an eye on her.