You can use BrokerCheck, a free online tool offered by FINRA, to review a broker's qualifications, registration, and employment history. BrokerCheck also contains a disclosure section with information about customer disputes, disciplinary events, and certain criminal and financial matters on the broker's record.
One sign of an unscrupulous broker is if they churn accounts (trade frequently) in order to generate commissions for themselves. Also to be avoided are brokers who recommend investments below breakpoints in order to protect their commissions.