Watch for mark ups on ECN fees.
Go to all the exchanges websites every month and check the fees; when fees are raised or lowered, you want to make sure it's accurately updated in your favor. Go through the tag on every fill and make sure it's proper. You can't really know until you open the account, but when you do, go through the clearing report line-by-line, execution-by-execution, to make sure everything is legit. Are you getting .24 rebate for posting BATS? Did you get a charge, instead of a credit, for a resting order?
What happens is 95% of traders aren't aware of what the real fees are, and will foolishly get screwed over. If you catch a "glitch" in the clearing, they will say "oops" and fix it (at least for your account.......). The glitches are never in your favor and always against you.
Any kind of "other" fee as well... a firm can offer $1/1000 and then charge a $.30/1000 "other" fee, and call it whatever they want. Now your ticket is actually $1.30/1000 instead of $1.00. NASD, NSCC, TAF, SEC, and ECN fees... know what they are and make sure they're correct.
My mentor said that being a trader also entails being your own accountant. He's very much right.
I would say pretty much everybody will try to play games, no matter how great of a salesman the firm's owner is, or how reputable or old the shop is, you have to be on top of them OR THEY WILL GET YOU.