6-APB has been one of the substances where a lot of other stuff has been sold in its name (like ecstasy). This is why it is important to do allergy tests (here also meaning low dose test for improper compounds).
A basic allergy test procedure is:
1) Take approximately 0.5 mg of substance (by volumetric dosing - or as a less safe alternative: the SMALLEST speck of powder you can find) and ingest it, preferably sublingually (i.e. keeping under your tongue for 15-20 minutes), as this will also show whether you have received a highly potent but not orally active compound (like the NBOMe-class). Alternatively, but less safe, look up ALL the compounds a vendor may be selling and find out the dose ranges for each, and use the lowest one as a starting point. If you detect ANYTHING at an allergy dose level, DO NOT proceed to the next step.
2) If you don't detect any activity or unusual symptoms (i.e. rashes, reddening of skin, trouble breathing properly), then go on to about 25% of an active dose (25-30 mg in the case of 6-APB) in order to see whether the compound shows strong activity at this point - which it shouldn't, but there might still be another chemical in it, such as a 2C-X (doses between 8 and 50 mg depending on which one it is). If you detect significant activity (i.e. a "full" trip") at this level, do NOT continue to the next step. Be advised that 25% of an active dose of a psychedelic, and especially an entactogen (MDMA, 6-APB, MDAI) will still produce non-insignificant tolerance and you should wait a few days in the case of psychedelics and a few weeks in the case of entactogens before proceeding to step (3). This step is generally useful, incase you receive 2C-P instead of 2C-D, or MDPV instead of methylone. Vendors sometimes mislabel products and this has led to several deaths and hospitalizations; be safe!
3) In case step (2) went okay (i.e. only mild effects), you can proceed to a normal dose.
Bonus tips:
When doing step (1), look at ALL the vendor's compounds, and be aware of the fact that it is a possibility that you may end up intoxicated for as long as the duration of their most powerful substance - in some cases over 24 hours of tripping (DOx) or 10-15 hours of being stimulated (2-DPMP). I.e., don't do allergy tests the day before you have do something important - just in case
More detailed info on volumetric dosing here:
Safer Research Chemical User's Guide. Remember: it is IMPERATIVE that you weigh the initial amount before dissolving it into a liquid - vendors have regularly been known to be generous when sending out chemicals.
I might have forgotten a single point or two - someone else will hopefully be able to fill in.
