The Russians probably did it just like the Brits say. Nerve agent poisoning would be readily diagnosed by depressed levels of acetylcholine estersse in the victims’ blood. A number of field and laboratory tests can identify the essential chemicals in the agent, such as phosphorus and sulfur (if it’s formulation is anything like VX for example).
However, Mueller made such a dog’s breakfast out of the Amerithrax investigation, and all of the false starts regarding WMD in Iraq that I think folks are rightly skeptical of pronouncements on WMD by government. The Russians are taking advantage of that skepticism.
The American anthrax attack was traced back to a US government bio-weapons lab in Maryland...go figure.