Culled from IMDb. The first time there was ever an intelligent post at that place!
We know that Sarah is mentally unstable, and we know that she did have other hallucinations, at the hospital and then of her daughter, so we've got a precedent. Also, take a look at the context in which her auditory hallucinations, the ones of her daughter giggling, happened.
-In the dream at the cabin, right before the pole smashed through the window.
-Right before we see a crawler – our first deliberately placed one, too. There’s one about half a minute earlier that’s extremely difficult to spot unless you know it’s there, but on the commentary it was explained that this was actually a member of the crew whom they only noticed during editing, and touched up his face so he’d look like a crawler. So it was intended that Sarah would hear the giggling right before our first crawler makes an appearance.
-Before she enters the tunnel, the one she got stuck in. As Beth pointed out, Sarah could move, she wasn’t genuinely stuck, it’s more that she was panicking and convincing herself she was stuck. It was a situation her mind created.
-While everyone’s busy with Holly’s leg, Sarah hears the giggle, wanders off in the direction it came from, and comes across a crawler.
This is probably the biggest piece of evidence for the crawlers being Sarah’s hallucinations. I mean, it’s difficult to come up with an entirely plausible explanation as to why Sarah twice imagines a giggle just before a crawler pops up. Especially since the other two giggles come right before Sarah is subjected to an imagined danger.
Another interesting little thing – when they’ve just entered the cave, right before she stumbles upon the bats, Sarah finds those bloody indentations in the rock, that look like somebody made them with their fingers. But thinking about it, how would the crawlers being real explain that? They wouldn’t be able to scratch into boulders. That bit only really makes sense if those indentations are just the first part of Sarah imagining the crawlers.
Oh, and also, there’s that dream of Jessica turning into a crawler. That could represent Sarah’s daughter-issues developing into her imagining the crawlers.
Also, if someone mentally unstable were to start having full-on hallucinations of monsters trying to eat her, it’s quite possible that it would have to do with some distressing factors. Sarah’s spending time in the dark under a whole lot of rock and dirt in a very claustrophobic environment, and that’s before things start going wrong. Then she becomes trapped down there, without Search and Rescue on the way, so it’s looking fairly likely they’re all going to die down there. Then Holly breaks her leg, and of course then Sarah’s crutch Beth is kind of busy helping the woman with the broken leg.
Then there's also the matter of how the crawlers were appearing to or with Sarah for a long time before anyone else. There was that one round the corner from where she was, to the left of the screen, in the first chamber. Only she was onscreen at the time. Then the next one only she noticed although everyone else was in the same chamber at the time – this was in the scene after the cave-in. Then there was that one she found while the others were fixing Holly’s leg, she got a reasonable look at it, and then it scampered away, conveniently right before Juno got there. Even when one appears to everyone for the first time, Sarah's the first one to see it, through Holly's camera. All this is a little odd, huh?
The gradual buildup is also interesting - the first one is barely visible even if you know it's there, the second one is only silhouetted and far off, and to the far left of the screen, the third one is much closer and centered, but still silhouetted and only there for a moment, the fourth one is in close-up, but almost entirely offscreen, and finally the drinking one is where we get a relatively good look at it. This could represent how the idea of the crawlers was gradually being constructed in Sarah’s mind. Notice the crawlers also get more prominent as the caving situation worsens.
Now, here's where the evidence given in the above two paragraphs gets fun – the main character sensing the threat before the other characters is a fairly common thing in these kind of “group of people getting picked off one at a time,” horror movies. The gradual buildup to the threat is also often used. So we can’t actually be sure – does Sarah see the crawlers so long before everyone else because she’s actually imagining them, or is it simply because she’s the main character in a horror movie? Is the gradual buildup to the crawler reveal representing the idea of the crawlers slowly taking root more and more firmly in Sarah’s mind, or is it because building up to the threat is a classic horror tactic? The film is deliberately taking advantage of horror formula to keep things ambiguous as to whether Sarah’s crazy or the crawlers are real.