What I would do is check your broker's spreads against what's offered in comparable futures contracts on the CME/IMM.
My bias is always towards open crossing markets. At least with such markets, the prices offered are guaranteed to be real (well, at least close to guaranteed, I think). Firms can theoretically "bait-and-switch" OTC contracts, although I have no idea how much of this actually goes on today, if any.
The fact that an ECN-like system hasn't developed for spot forex seems like an opportunity to me for the players in this space (NYSE ARCA, NASDAQ, BATS, CME....), especially on the seemingly successful make-rebate/take-fee model; but I'm probably missing something about the complexities of implementing such a market for spot forex.