forex? futures? stock? options? US based? You can probably start with anything the SEC/CFTC/NFA says about them, the NFA, Their financial statements, etc. Research them on the web, BBB.org Since you do not even mention them here, it is hard to get any feedback from ETers...
Its a well rated firm on ET, so I didn't want to mention their name. Trading stocks. My research on the net indicated that private firms have to disclose to sipc but not to public. I was hoping some one could supply a specific answer.
I'm assuming you're talking about Lightspeed. You can check out their SEC filings here: http://sec.gov/cgi-bin/browse-edgar?action=getcompany&CIK=0000916916&owner=exclude&count=40 If it's not Lightspeed, you can use the search feature and might be able to find some of the filings.