How could Chihiro tell that none of the pigs were her parents?

methroach wrote:

In fact, Haku told her to remember what they looked like. She later dreamed that she would not be able to distinguish them from the other pigs. But at the end, she was able to tell that they weren't part of the group she was shown because she grew up and gained confidence in herself during her adventures.

She got told by Haku that if she forgot she wouldn't be able to help her parents and would get stuck in the world of spirits, so she always tried to remember what happened. That's how she could tell.

Kind of conspiracy but it makes sense. Here it goes: I think that Yubaba (the witch lady who came up with the challenge) didn't know the difference either. By putting pigs that were recently turned into pigs, she guarantees that none of the pigs are Chihiro's parents, and then that nullifies her spell (which states that if Chihiro can tell which pigs are her parents, they can all go home human). That is supposed to make things easier for herself; but all along Chihiro knows its impossible to know which pigs are which, and therefore knows that the witch won't risk something as stupid as putting random pigs together, because the witch herself doesn't know which pigs are her parents. Its complicated, but basically boils down to this: if you figure out the secret to the spell (none of the pigs are her parents), the answer becomes invalid (which one of the pigs are her parents) and thus you deserve the prize (the pigs in question are human again).

I think in the earlier scene when Chihiro's parents first eat much at the restaurant, they have some kind of hairs left on their head. Meanwhile, the pigs were chosen by Yubaba seem had no hair on their head. But of course it's also a family bond I think

For me, Since haku told her to not forget who she is so she may include how her parents looked like ... And her parents weren't pigs in the first place so it's understandable that her parents are not one of those pigs

According to Rocket News 24, this is what Miyazaki himself answered a few years ago:

“I’ve never explained why Chihiro knows that her parents are not among the group of pigs towards the end of the film. Those people who are constantly seeking explanations often say that it’s illogical. However, I don’t think those kinds of things are important. After all the things she’s experienced up to that point, Chihiro simply knows that her parents aren’t there. You ask why she knows, but knowing is human life. That’s all it is. If you can point out that something is lacking here or there, then the audience should fill in the gaps for themselves. I don’t want to waste time thinking about those kinds of things.”

More recently, again from Rocket News 24, a Studio Ghibli employee said that Chihiro could tell because she had obtained special abilities in the spirit world:

“Chihiro, as a 10-year-old girl, could understand the difference because she had overcome difficulties and had managed to acquire the ‘energy to live’ – which is something everyone can do naturally”.

I think logically this is what happened:

Yubaba uses the test as a last attempt to not let go of Chihiro and her family. Logically she does not know which of the pigs are who anymore as there are no tags or distinguishing features. There are only 2 options: Chihiro got answer incorrect or not. So Yubaba chose some of her workers in the bathhouse and turned them to pigs. If Chihiro does not answer correctly Chihiro and family stay and she can just turn the pigs into workers again. If Chihiro is right the workers will be back and only her parents will be turned into humans (the other greedy humans will stay pigs).