First, you can review the comparison between the broker's swap rates and the actual differences between the central bank's benchmark rates. If the comparison is not too drastic, then it can be concluded that your broker doesn't manipulate the swap rates.
However, if it is too extreme, this could be a sign that your broker is taking too much profit from overnight positions.
Traders will be able to take advantage of carry tradesIF the broker applies swaps in accordance with each country's central bank benchmark rates. Which is actually unlikely.
Plus, terms and conditions on several brokers clearly spelled that they will hand you negative rates on overnight trades (add that with: no matter what the rate difference between currencies). Or alternatively, they hand you the negative rates but pocket the positive rates.
Withdrawal issues are the most common problems with brokerage companies. On the other hand, making deposits usually comes smoothly because no one will deny taking your money. That is why it is crucial to check a broker's withdrawal and read the user reviews on that matter before opening an account.
Slippage: You should be cautious if the slippage happens very frequently and ends up negative most of (if not all) the time.
Spoofing: It consists of developing a fake interest in a currency pair by placing phony orders on them without the actual purpose of trading that currency in the future.
False spikes: Unethical brokers make use of this feature by fabricating price surges in order to trick unsuspecting traders into placing orders.
Stop hunting: They bring the prices to a point close to the stop loss order that forces you to exit the trade, after which they push the prices to move in the opposite direction.
Front running: When a trader indicates that they intend to make an order, the broker immediately places their own order to profit from any changes in pricing.