How do I know if I can trust my forex broker?

There are a couple of ways to check up on your forex broker. For the most part, these steps aren't much different than what you would do with any other financial services professional that will provide services to you critical for your well-being.
One nice thing about choosing a forex broker as opposed to choosing a stock broker is the lack of frequent conflicts of interest. Even with all of the regulatory power of the SEC, seeing stock brokers and Wall Street firms found guilty of an almost casual abuse of their clients' trust is almost a regular event these days. The forex market is so large, however, that it is hard to profit off of someone else's positions, even for the especially large banks.
First, check the website out thoroughly. The forex market involves huge sums of money, and is a place only for professionals. If a broker's site does not suggest a total commitment to perfection and customer service, as well as a total intimacy with technology, don't bother.
Second, check with the National Futures Association, or NFA, to see if he is registered as a broker. There is little reason to deal with any broker who is not registered with his industry's trade group.
Third, check his references and his resume. More important than the length of his resume – assuming he's not an amateur – is a certain sense of perfectionism displayed in his professional endeavors.
Lastly, do not be overly concerned about fees. Remember, your forex broker will, in the long run, hopefully make you a significant amount of money. To be sure, there are certainly a large number of brokers who charge first-rate fees and do not give first-rate services. Certainly, you want to avoid getting ripped off. However, it is much better to receive good advice that's slightly overcharged than it is to receive bad advice for a little less money.