In the Mistborn series, we know that an occasional Skaa will have some sort of allomantic power, and I assume that either they are discovered by a bronze seeker or their power is self evident. Copper burning "smokers' can't be found by seekers because their ability is to hide allomancy, yet somehow smokers find each other for training and some thieving crews, like Kelsier's, hire a team of smokers to cover their tracks. I feel like this is a big contradiction! How are smokers discovered or how do they even discover themselves if their only ability is to hide other's allomancy?
This isn't documented anywhere in the plot, but it seems fairly simple - they're also detected by bronze Seekers. Either by bringing along another Misting to see if that Misting's powers are obscured, or by testing the Seeker's own power within the Smoker's cloud to see if they can sense at all, would tell them what they need to know. They would not be able to directly detect a Smokers power as you note, but they could easily tell that their own sensing powers are affected, and there's your successful copper test.
Since you also ask how they get organized, keep in mind they're being sought. Snapped but untrained Mistings may not even know what they can do or how to find more information. At most they can only ever burn trace elements (see Vin's example), and with copper there will be little to no effect from that, even moreso than with other Misting. That said the skaa underground is quite organized, and has a vested interest in finding such people, whether their business is criminal activities or the more legitimate resistance figures. Even apart from Kelsier's little band, Clubs has built an entire successful business front out of hiring out himself and his Smokers. He and others like him would keep in touch with anyone who might have talent, and they know what to look for - half-bloods produced from the Houses that like abusing their Skaa, unexplained sensations or hints of accessing metal reserves however small...
After re-reading the series, I think I have a better idea how this works.
Copper mistings have essentially 2 abilities:
- The power to obscure allomancy.
- The power to resist emotional allomancy.
This second ability is the one that is missed in the question. This ability is something that would provide a personally observable benefit. If the allomancer knew about a soother or a rioter, and noticed that they could resist it where others couldn't, then that would be a clue into their own abilities. Likewise, a soother or rioter might notice that someone was not particularly easy to manipulate, and that would prompt further investigation.
Later in the series, we get a look at some recently snapped mistings realizing their power for the first time. When they swallowed metals, they suddenly felt a well of power inside themselves, even if they didn't immediately understand it. If a copper misting were to get a particularly large amount of copper in something they ingested, they would recognize the power and likely trace it to allomancy.