I'll weigh in with my two cents to give jcl a hand:

a) Some brokers will actually supply you with a receipt for each trade which includes the details of the liquidity provider that took your trade. One such broker here in Australia is Global Prime. I use them for some of my automated trading and can recommend them highly.

b) You could always run two accounts side by side and compare the ticks you receive. But maybe your broker is aware of this possibility and gives the same ticks to the same IP address.... I am being slightly facetious, but the best way around this issue is to trade with a broker within a regulated jurisdiction. I know that the brokers here in Australia generally value their licenses extremely highly and would make a lot of effort to play by the rules. If you have a concern, you can lodge a formal complaint with the regulator (ASIC, FOS here in Australia) and be assured that it will be followed up. Trading with an unregulated broker or in a questionable jurisdiction means that you have to take the broker's word that they are playing by the rules. You can always choose who you trade with.

c) Not really. A broker's price structure isn't dependent on whether or not it is DD. The two price structures I am aware of are fixed spread, and variable spread + commission. What works best will depend on your strategy, to an extent. I would avoid DD not because of the price structure, but because of the fundamental conflict of interest.