Long Version: So typically it's not really "mixing" or "cutting" with LSD. What is most common with LSD is laying 25I-NBOME on a sheet and selling it as LSD. The same is done with DOX and innumerable other research chemicals. These are both relatively untested research chemicals that produce similar effects but have more potential side effects. They use these because you can (kinda) trick someone into thinking they're taking LSD because it is a trippy experience and you're unlikely to think it was not LSD. The issue with these drugs is that they kill people so one ought test their "LSD".
LSD along with DMT and mushrooms is an indole alkaloid which means (roughly) they have a shape that can fit into a serotonin receptor nicely which is what gives you the effect of tripping. So, if we can test for them we can tell if it's really LSD or some shitty analog.
BUT HOW?
Thankfully there exists a test for just this. It's called an Ehrlich reagent test and you can buy one for about $15. https://dancesafe.org/product/ehrlichs-reagent-testing-kit/
To learn more go here: https://www.youtube.com/watch?v=1g0KThAbkjU
Edit: Additionally, buying online helps reduce your chances of getting some shitty research chemical because: 1. A vendor typically shoots for having repeat customers. 2. If you get LSD that is not LSD, you can leave a bad review saying so. But still get a test kit.