By now, any meth users with more than half a brain left (admittedly, not the highest percentage) have figured out that there's a lot of crap out there, especially on the darknets. It looks virtually identical to methamphetamine, and forms very large, very clear crystals, but it appears to be isopropylbenzylamine with just barely enough methamphetamine in it to keep people coming back for more. It's the inevitable result of tweakers using crystal size and clarity as a way of measuring purity. Isopropylbenzylamine is very easy to grow into large crystals, whereas real methamphetamine is not. If a darknet vendor is advertising the size and clarity of their crystals, rest assured, that shit is faker than a 40-year-old stripper's boobs.
So, how do you test it? If you have enough of it, the best way is probably the melting point test. It's a common way to test the purity of any substance that's supposed to be fairly pure. Get a glass thermometer than can measure up to at least 200˚C, a test tube, and a vial or a small beaker or something, that you can put the test tube in without it falling over. Put as much of the alleged drugs in the test tube as you can. Don't worry, it's a nondestructive test. Put mineral oil or cooking oil or some other clear liquid with a high boiling point in the beaker, then put the test tube in that and heat it on a stove burner or a flame or something. Eventually, the stuff in the tube will melt, and when it does, insert the thermometer. Wait for it to all melt, then turn off the heat and start monitoring the temperature. As soon as it starts to form crystals, make a note of the temperature. Pure methamphetamine should begin crystallization between 170˚C and 175˚C, and with cut methamphetamine this would happen at a lower temperature. If it's pure, this temperature should not change much at all during solidification. If it's highly-cut, then this number will be about ten degrees lower by the time it's fully solid. If someone sells you pure levo-meth or something, you can even test it by mixing with some known dextro-meth and seeing if this changes anything with the melting point. If it does, that means that you now have a mixture and not a pure substance, since the two substances weren't identical.
The Marquis test sort of works, but isn't super reliable because most of this stuff does contain methamphetamine, and n-iso shows no reaction to the Marquis test.
If you don't have enough for a melting point test, dissolve as much as possible in a small amount of distilled water, then put a few drops on a smooth flat surface, like a mirror or a plate. N-iso takes forever to evaporate, and when it does, there will be a geometric-looking crystal left behind, that's wet in the middle when you crush it. Methamphetamine will leave behind white crusty stuff that only consists of small crystals, and will evaporate considerably faster. If you're in a hurry and don't want to wait overnight, or however long it takes n-iso to dry, you can dissolve it in at least 90% isopropanol rubbing alcohol instead of water. Again, large flat crystals means n-iso, small crusty ones means methamphetamine.
Okay, so it's cut, now what? Well, it's very, very difficult to separate them, which is the whole reason this bullshit became popular for cutting. Despite claims on other message boards, chloroform doesn't work very well for separating them, and the best it can do is concentrate them somewhat. These two substances only have similar properties in the form of their hydrochloride salts though, and are less similar in other forms. If you have multiple ounces of this crap, I'm sorry to hear that, dude. You can separate them by vacuum distillation of their free bases. N-iso has a higher vapor pressure and lower boiling point, so it will come over first. If you want, you can just separate the two this way, then add hydrochloric acid to the higher-boiling fraction, which will probably be depressingly small. Then just recrystallize that once or twice to get rid of the excess HCl. This way minimizes mechanical losses, so it would even be possible with as little as ~10 grams or so.
If you're not a chemist, you can separate them based on the fact that the freebase of n-iso evaporates more easily. Get a test tube or a narrow vial or something, put a bunch of your shit in it, then put some potassium hydroxide flakes on top. Yes, sodium hydroxide can also work, but not as well, and you'll lose more, so if you can afford methamphetamine, you can afford potassium hydroxide. Heat it until it starts to melt, and they'll react, producing an upper layer of n-iso and meth, and a lower layer of water, KCl and KOH. If there isn't enough liquid, you can add water by the drop until there's a good separation. Heat to make sure the substances have reacted completely. Keep adding solid KOH until it doesn't seem to be reacting anymore. Aim for a lower layer that has both solid and liquid in it when it's cool. The upper layer will be entirely liquid. At room temperature, n-iso will be a thin, clear liquid, where methamphetamine will be more viscous, and translucent. So just let it sit at room temperature until the physical properties resemble methamphetamine more than n-iso. Since you're not a chemist, don't fuck around with it to try and speed things up. You're only going to spill it or some shit. You can take it out with an eyedropper to sample whenever you think it might be done. Some people prefer the freebase, but if you prefer the HCl salt, then figure out how to get it yourself. If I have to explain it to you, you're just going to wreck your teeth.
why not just do a bleach test it's so much less work.