Yeah, you can see if it's discolouration (albeit it's easier to tell in person than on a photo since photos are subjective when it comes to lots of things like lighting, screen pixels, etc). Sparkle pony discolouration tends to stand out. My store Twinkler has an uneven colour issue due to discolouring. Can you see it in this photo? It's the best immediate example I have.
visitors can't see pics , please
register or
loginThe one that is often the most obvious is MO Napper, because she can go full scale brown and that's definitely NOT normal or a batch variation. On the other hand, MO Napper is more yellow than her store counterpart anyway, so if you were to compare store Napper and MO Napper and find the latter to be more yellow, it would not mean she had discoloured.
I think it's just something when you see it you know that's what it is. Which is an unhelpful answer but the best I have.
On the subject of yellow moondancer vs a yellowing old Moondancer.
Yellow Moondancer looks like she's made of cheese.
It is really easy to tell them apart. She's just MUCH more actually yellow that you can tell it's intended.
visitors can't see pics , please
register or
loginMy regular Moondancer looks ok in that image because of the contrast BUT...
visitors can't see pics , please
register or
loginHere's a comparison including the retro Moondancer as well and you can see that she's not.
And a good example of how batches can actually influence discolouration using Starshine.
visitors can't see pics , please
register or
loginvisitors can't see pics , please
register or
loginYou can tell they're from different production batches because, the colour issue aside, the symbol glitter is applied differently (Second one the symbol has damage) and the dark pink hair tone is slightly different in shade. There is also a difference in the plastic generally though. So even though we think of Starshine as a pony who commonly discolours, the fact is that it is not universal.
Cherry Treats when she discolours tends to go tan, NOT yellow imo. Albeit it seems more common on the versions made in HK, rather than China. I have heard of Chinese ones discolouring, but have never seen one in person (and that's the default version here in the UK). My childhood one is still snow white, as is the other I have, but then so is my HK one. The difference when you look at them together is that the HK one has a more translucent type of plastic - which may be more predisposed to colour issues.
Finally, people like to say Twist and Skyflier have a white streak in their hair. But both my MOC ones have pale pink streaks. The thing is it's hard to know if that's the intention or if it's somehow been coloured by the red in their hair. My loose ones also have pink streaks. If I separate the streak from the mane and put white paper underneath it it is definitely pink, and not white. But you will struggle to find an ID site- or even card or box art - that agrees with that.