Be especially wary if you've made a bad investment before. Ask what the risks are. There is no sure thing all investments involve risks. If anyone tells you differently, walk away. Ask about commissions and management fees.
Fees vary depending on investment, and it can pay to shop around.
Be wary of investments that have no fees. Ask the caller to send you a prospectus. Legitimate brokers will be happy to send you one. And they will be willing to wait until you have read it. Don't buy the story that the prospectus is at the printer.