They can see if the order was submitted via an EA, as part of the order message sent to the server includes both an EA number and order "comments" (which a lot of EAs use.) That doesn't mean they know the specific EA you're using, just that it's an EA submitting or modifying the order. To detect nasty EAs they don't want running on their feed, a broker will go a step further and match the execution patterns of your account against known profiles of bad EAs. An example of this would be latency arbitrage.. so seeing an EA submit and then close orders with an average hold time of just a second or two, with a hit rate north of 90%, would probably trigger a red flag and call the account up for review. (In this case, if the broker is A-booking your orders, they don't actually care that you're making money, they just care about protecting their relationship with their liquidity providers.. or if the broker is B-booking, then they care about losing money.)