This might be a bit hazardous to say, but in my experience with buying cheap flash devices from questionable sources (eBay, Amazon, and AliExpress), I've had remarkably good luck. In fact, in around 20 total devices, I have never received a fake. I thought I did once, when I got a 32-gigabyte pendrive for around $20 that didn't enumerate, but it turned out the flash was good - it just had a really janky USB connector that didn't really work in almost any of my computers. Rules I've followed that appear to work:
1. Avoid comically large capacities. 16-gigabyte cards are probably fine, because 16 gigabytes of flash is so cheap that it's probably not worth faking it. Meanwhile, I'd be surprised if the majority of 1-terabyte cards were legit. I usually don't consider anything above 32 gigabytes, as a general rule - both to avoid fakes, and also just because I don't like investing tens of dollars in a chip that's smaller than my fingernail and might easily get lost in my absurdly messy shop.
2. On sites like Amazon and AliExpress (which I really believe to be about comparable in overall trustworthiness, despite typical American opinions about "cheap Chinese crap" - most of it comes from China either way), it's really not worth sifting the list to find the lowest price. It's much better to find a seller that has a brand name, a lot of listings that have stayed up for a long time, and a lot of customer reviews with relatively even distributions in time. The usual scam techniques are to put up a burner storefront that only lasts long enough to make a few sales (and possibly pads its own reviews with bots, but I haven't actually seen this), then disappears before it can be killed by the reviews.
3. Don't get distracted by high numerical spec-to-cost ratios, just in general. You don't want the item with the best advertised specs, nor do you want the one with the lowest price. Look for something with perfectly average specifications and a price similar to what other sellers are offering, and get that. I know how hard it can be to convince yourself of this, if you've spent a lot of time comparing things in brick-and-mortar department stores, but I'm serious here: just try to hit the center of the bell curve and ignore the tantalizing tails. If a product doesn't look that exciting, that's probably because it's being advertised more honestly than the ones that do. This is increasingly true in a tech market like the one we have now, where the growth speed is starting to slow down a little bit and the playing field between competing vendors is starting to level.
The only fake flash devices I've ever encountered have been little promotional flash drives I got as freebies at a parks department event near here. They claimed to be 4GB by their casing markings, but the partition table onboard had been hacked to claim it was several hundred terabytes (which I can only assume was a mistake on the counterfeiters' part). The actual capacity, visible once the partition table was wiped clean and properly recreated, was 4 kilobytes. I think I got relatively lucky here, since they're still actually viable, if very small, flash devices - had they been the firmware-hacked variety that report a false capacity at a deeper level, they'd be impossible to use safely without hacking the firmware back into line.
Addition: It's also worth noting that smaller cards are less likely to be fakes because they're easier to test.
A testing a 16-gigabyte card requires 16 gigabytes of test data, which is relatively easy to generate and handle. Testing a 1-terabyte card requires one terabyte of test data, which is rather a lot, and most people won't have the free storage space (I know I don't) or the time to complete a proper test. When you can identify a fake in a few minutes with your laptop, it's much harder to get away with shenanigans compared to when it takes hours and a proper workstation.
1. Avoid comically large capacities. 16-gigabyte cards are probably fine, because 16 gigabytes of flash is so cheap that it's probably not worth faking it. Meanwhile, I'd be surprised if the majority of 1-terabyte cards were legit. I usually don't consider anything above 32 gigabytes, as a general rule - both to avoid fakes, and also just because I don't like investing tens of dollars in a chip that's smaller than my fingernail and might easily get lost in my absurdly messy shop.
2. On sites like Amazon and AliExpress (which I really believe to be about comparable in overall trustworthiness, despite typical American opinions about "cheap Chinese crap" - most of it comes from China either way), it's really not worth sifting the list to find the lowest price. It's much better to find a seller that has a brand name, a lot of listings that have stayed up for a long time, and a lot of customer reviews with relatively even distributions in time. The usual scam techniques are to put up a burner storefront that only lasts long enough to make a few sales (and possibly pads its own reviews with bots, but I haven't actually seen this), then disappears before it can be killed by the reviews.
3. Don't get distracted by high numerical spec-to-cost ratios, just in general. You don't want the item with the best advertised specs, nor do you want the one with the lowest price. Look for something with perfectly average specifications and a price similar to what other sellers are offering, and get that. I know how hard it can be to convince yourself of this, if you've spent a lot of time comparing things in brick-and-mortar department stores, but I'm serious here: just try to hit the center of the bell curve and ignore the tantalizing tails. If a product doesn't look that exciting, that's probably because it's being advertised more honestly than the ones that do. This is increasingly true in a tech market like the one we have now, where the growth speed is starting to slow down a little bit and the playing field between competing vendors is starting to level.
The only fake flash devices I've ever encountered have been little promotional flash drives I got as freebies at a parks department event near here. They claimed to be 4GB by their casing markings, but the partition table onboard had been hacked to claim it was several hundred terabytes (which I can only assume was a mistake on the counterfeiters' part). The actual capacity, visible once the partition table was wiped clean and properly recreated, was 4 kilobytes. I think I got relatively lucky here, since they're still actually viable, if very small, flash devices - had they been the firmware-hacked variety that report a false capacity at a deeper level, they'd be impossible to use safely without hacking the firmware back into line.
Addition: It's also worth noting that smaller cards are less likely to be fakes because they're easier to test.
A testing a 16-gigabyte card requires 16 gigabytes of test data, which is relatively easy to generate and handle. Testing a 1-terabyte card requires one terabyte of test data, which is rather a lot, and most people won't have the free storage space (I know I don't) or the time to complete a proper test. When you can identify a fake in a few minutes with your laptop, it's much harder to get away with shenanigans compared to when it takes hours and a proper workstation.