33 Saun 717
For something like this, F'mos did not even need any fancy means to detect the magics in the dust. All he had to do was keep an ear out for the weird occurrences which followed the unnatural lightning storms and there, the alchemist would find the reagents he wanted. It would have been easier for him if he started in the earlier trials of the season but as no one even figured out the correlation between the storms and the magic freak accidents, F'mos was not even aware of the wealth of material which made its way to Etzos. By the time he actually started there were plenty of other hopefuls like him out to profit from the coloured mists; unneeded competition for him.While he was behind them most of the time there were enough times which F'mos came out with the prize he wanted to make the effort well worth it. In one of the first times he chased after the storms, the bright flash of light from the sky landed right in the forest beyond the walls. Not too far off from where the chef worked. Because the Blacks have yet to notice their employee's interest with the storms, F'mos easily persuaded Maylan to let him to into the forest where Rowan hunted.
As for the storm, sure, you could possibly see lightning strike a time or two, and see some weird residual sparkle or colored mist or something. You can collect whatever it is that is giving off this effect for later use.
Let's say one "color" of this effect is an inhibitor, which could go so far as to empower wards, and the other is an accelerator or metabolizer, which will enhance the magical effect of something. We can work out details later.