In layman's terms, what would you say reagent tests are good for?
They are generally intended for gaining insight into what a substance most likely is and confirming the absence of common harmful adulterants. Also, while writing this out I realized that you could use Simon's reagent to confirm that your sample does not contain cathinones such as the 3-FMC that I said it could contain.
For example, if you use Ehrlich reagent on an LSD blotter and it turns purple, you know that an indole alkaloid is present. Indole alkaloids could be 5-HTP, LSD, AL-LAD, DMT, 4-AcO-DMT, 5-MeO-DMT, etc.
All of the indole alkaloids that could fit on our blotter at active dosages are lysergamides. Some lysergamides that are prodrugs for LSD (1P-LSD, ALD-52) will react very slowly (several minutes or even hours) or not at all. So that means that our tab could only be LSD, AL-LAD, ETH-LAD, etc.
Since blotters almost exclusively contain a singular chemical we do not need to use another reagent to test for NBOMe or other harmful compounds commonly sold as acid. They could in theory contain active amounts of fentanyl or carfentanil, but I have yet to see a single case of these substances occurring alongside LSD.
The other example I'll give you is ketamine. We'll start with Mandelin. Our sample should turn orange. We know that we have Ketamine or 2-MeO-Ketamine We can rule out PMA/PMMA, DXM, 2F-Ketamine (would not react), MXE, and the PCE/PCP analogues. Cocaine can be ruled out as long as the colour doesn't return back to yellow after turning orange.
Next, we move on to Mecke. We get no reaction, indicating Ketamine/2F-Ketamine/O-PCE. 2F-Ketamine or O-PCE would not have reacted with Mandelin, so at this point we can be pretty sure that we have ketamine. No reaction with Mecke also rules out many cutting agents. Simon's reagent can be used to rule out most cathinones. Marquis can detect secondary alkaloids in Cocaine if Mandelin was not clear or if you are worried that your sample contains a mix of both. These tests all give us the presence of ketamine and rule out MOST adulterants. You can use fentanyl test strips to rule out fentanyl/its analogues. At this point you aren't positive that your ketamine isn't cut or is of good quality, but you can be pretty damn sure that it's present and that essentially all common harmful adulterants are not.