Aside from subjecting the source tabs to the 'acid test', as it were, you would use some lab testing to verify a purchase such as on the DNMs. EnergyControl if you're in the US, possibly some of the national testing programs if you're in the EU. (EnergyControl was not available when I ran my self-experiment.) Another possibility for quality control is moving to the gray market: the analogue 1P-LSD is available from RC vendors and currently is minimally prosecuted, although it's so chemically similar to LSD that it'll probably happen eventually; to the extent that you trust legal sellers more than illegal ones for purity, 1P-LSD would be an improvement. Or you could do both, why not.