I bought one-off packs of both the Legacy and the Modern set from a Chinese vendor (I'm way too skittish about fakes to not have held some myself), and I can say this:
There are different print runs. The Modern set I got was from an early-2014 run, and they were incredibly glossy, almost sticky to the touch. On top of that, the colour was off on most of them - Goyf, Grove and Canopy stuck out to me as ones where the colours were very obviously visually off. Some also had obvious mistakes, like the mana symbols on Vendilion Clique and Goblin Guide being way off in both the colouring and the shading and, most hilariously, a Sunken Ruins that tapped for a blue-tinted tap symbol. A few of them were good enough to pass on just a visual inspection, though - most notably Liliana of the Veil. She was still glossy and sticky as hell, though. The Legacy set was from a mid-2015 run, and it was significantly better, but it also had numerous issues, which leads me to...
The older cards are too perfect. The dual lands, the Karakas, the Wastelands, all of these were more than passable on a visual inspection. But if you have ever held genuine copies of these cards, you know that they feel different - Legends card stock feels a lot thinner and flimsier than new cards, and Revised duals feel thicker and bulkier. These all felt the exact same, no real difference. And I have seen very few Wastelands without some kind of chink in them, mostly owing to it being practically worthless for a long time - these were absolutely perfect, which is certainly weird.
As you noticed, some of the cards have very obvious mistakes that can easily be found on a visual inspection, even accounting for colour differences. Of note: The Sunken Ruins I mentioned earlier, Batterskull without the Phyrexian watermark, Force of Will has the wrong shading on the Alliances flag, Emrakul has an expansion symbol that's way too small, Ulamog's is way too big, the cog symbol on Urza's Saga cards is wrong, the morph spider on Onslaught fetches is extremely off, with the size being too big and the middle legs being raised about the side legs. And both types of Goyf, the MMA ones and the FS ones, are much too bright in colour compared to the real deal. It stands out instantly.
Of course, that's just on a visual inspection. If you're actually trading for them, they don't pass the blacklight test, they let through much less light than genuine cards, and the cardstock is much too good and fresh for something that's supposed to be old. I wouldn't know if they pass the loupe test, as I'm too lazy to get the implements for that, but my guess would be no. They actually pass the bend test way better than normal cards, which is weird. But anyways, if you're trading for them, just get them out of the sleeve and have a look. There's a lot of misinformation whenever this stuff is brought up, and I urge anyone who's gonna be trading or buying cards to educate themselves.