Reagent tests are cheap and people over-read them. Here is the honest version.
What they do: give you a colour change that is consistent with a class of compound. They are good at catching a gross substitution — you thought it was one thing and it is another family entirely.
What they do not do: tell you purity, tell you dose, tell you whether the thing you expected is the thing you have if two compounds share a colour, or detect anything at low concentration. They also do not survive well; a reagent bottle open for a year is not the same reagent.
So: reagent test rules out some disasters and rules in nothing. If you need a number rather than a colour, you need a lab, and a lab means money and a sample you are willing to lose.
This is right and worth repeating: a reagent test is a screen, not an assay. People post a colour and announce purity and it drives me mad.
Do the labs that take posted samples still exist? I thought most of the good ones closed.
A few remain, mostly harm-reduction funded and mostly national. They change names and funding constantly. The ones that survive tend to be the ones with a public method published.
And a lab result is a result for that sample, not for the batch. The next box is a new question.