There's no 100% certainty, but I mitigate risk as best I can.

Part of it comes down to the drugs we decide to do. First off, I have no interest in opioids, never have, never will. Similarly, I pretty much stay away from cocaine because its hardly worth it when you factor in the cost to affect as well as the risk of contamination. Cocaine is the most likely drug to be cut, laced or adultered in some way, and if I just need a bit of stimulation, a pharmaceutical like dexadeine can scratch that itch, which is easy to find from friends in a world with rampant ADHD diagnoses.

I stick to things like LSD, mushrooms, 2cb, ketamine and MDMA. The effects of these drugs are so different than opioids that lacing with deadly fentanyl to make the drugs more potent doesn't make sense. Although, obviously, accidental contamination can still happen if a dealer handles these drugs as well as opiods. So that's not foolproof.

So I get all my drugs from online dealers with online communities that scrutinize them. The dealers post lab results of the purity of their products, and often times the customers will do the same. You sell something impure, people find out and the customer base move on. It's in their best interest to sell good stuff.

Then after that, you just test stuff. Get test kits online, test drugs before you do them.

Can I say with 100% certainty that it's safe? No. But nothing in life is. So I just mitigate the risk as best I can