You can certainly ask a prospective attorney to sign a confidentiality and non-disclosure agreement (you can find them on the web) concerning the matters you wish to discuss, but I doubt that given the strict attorney-client confidentiality privileges that normally apply to such communications and the fact that you can get an attorney disciplined by the state licensing boards for unauthorized disclosure that many are going to be as "sleazy" as you fear.
If your case is that good (are you looking for a contingency case?), I doubt they will lie to you about its viability and then find some dummy plaintiff to sue who will want less of a cut. But the non-disclosure agreement should help if you don't retain them and they launch some class action suit on behalf of someone else as you fear.
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Attorney/client privilege generally starts when you meet with an attorney for the purpose of a consultation, which of course makes sense when you're talking about someone trying to get all the facts to figure out if you've got a case (or not). While that should be enough, you can certainly ask prospective counsel to sign a confidentiality agreement beforehand. But the best protection you can get is hiring reputable counsel with whom you are comfortable - and that is something only you can determine.
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