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...