Obviously importing regulated drugs may come with their own legal quandries. However, if you were to do such a thing, you would find information on what the pills and packaging should be, their ph, their coating and their weight.
Then when you receive you order, you'd sacrifice a pill to carefully weigh it, check the coating matches, packaging and so on. If you ordered a whole bunch, you would check batch numbers and sample each batch for testing.
Obviously this isn't foolproof, but if someone is going to sell a counterfeit they aren't going to practically spend a lot of time in making it a perfect one.
In many countries there are also testing labs that don't ask any questions nor contact the police if you hand over small samples of illegal drugs, for harm reduction purposes. If you wanna be 100% it contains nothing deadly, use those.
They come sealed from reputable manufacturers and pharmacies in India that you can read about online.
I suppose someone could create fraudulent drugs, but honestly, it's probably cheaper just to sell you the real deal. And just like other businesses they want your repeat business.
I know they are the real drug in my case because I take them for mildly high blood pressure. It's trivial to measure blood pressure, and I can see that they have the same effect as the American drugs of the same dosage.
How do you know? Do they have a lab certification/ is there a regulating body?