system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: how and by what means can the origin of the poison be determined, example from the recent poisoning in UK where they know it came from Russia?
gpt
Certain details of the manufacturing process leave detectable impurities in the chemical.  If you have samples to compare them to, you can determine with decent accuracy what lab they came from. The same sort of technique is used in drug investigations.

Also, particular ingelredients may be local - as someone else mentioned - making it possible to determine origin.

Edit: however, those sorts of things only tell you where it was *made.*  Russia has, at times since the end of the Cold War, been a *bit* careless with their military stockpile.  Presumably, this is why the Brits gave their unanswered 'deadline for explanation.' as opportunity for the Russians to explain the possible black-market movement of the chemical.

They may also have other types of evidence, intercepted communications or some cool spy shit, that indicate direct Russian involvement.