There used to be some darkweb marketplaces that were extremely reliable for psychedelics, for example "The Majestic Garden" was an old PHP message board which specialized in psychedelics and had a strict whitelist about what substances could be sold. TMG has since shut down unfortunately, but there are lots of other providers. Some "well-known" manufacturers used to hang around TMG, and if a listing on some other site mentions that it's their LSD then IMO it's a good place to start. General methodology is to get a burner laptop running some secure *nix OS (ie. Tails), acquire some Monero, navigate to an appropriate marketplace via Tor, and make your purchase. Use your real address and name for the shipping; it seems scary but in the US it's the most secure way (least likely to trigger a warrant being issued to search your package). Once that's done you head over to dancesafe.org on your normal laptop and order what they recommend for LSD reagent testing. Then you wait for everything to arrive, follow the instructions in the reagent testing kit, and if everything comes out "okay" then you're good to go.
In general it's fairly rare for someone to sell "fake" or "cut" LSD. This is for two broad reasons:
1. LSD isn't addictive in the same way as some other illegal substances, so there's much less profit in faking it. If you sell someone fake LSD and they have an awful trip they're never going to buy from you again and you've burned a profit stream. If you sell someone coke cut with meth they're probably going to come back anyways, because they're hooked on the drug.
2. The things you might adulterate or fake LSD with (ex. NBOMe) are also somewhat difficult and expensive to make, so it's not like one would really save much time or money. If you're trying to burn someone in an LSD sale, the simplest and most profitable thing to do would be to just sell empty blotter paper, rather than wasting research chemicals you could potentially sell to some psychonaut with the correct label.
With that said NBOMe and some other synthetics have been cropping up in the last 20-ish years. If someone at a party or festival is selling single tabs that's definitely the place where you might get burned with NBOMe, since by the time your trip ends the dealer will be long gone. In general it's always best to test everything you ingest, and dancesafe.org is definitely the place to get info on how to test stuff.