There are literally thousands of individuals that call themselves public speakers. Some of them are very good and extremely informative. Unfortunately, some of them are not very good and can be downright boring. So what should you look for before hiring a speaker for your event? Let me answer that question with another question. Would you like to be a passenger on a plane with a pilot that has never been in the cockpit before?
The first thing you should look for is experience. How much experience does the speaker have on the topic of his (or her) presentation. Then look for speaking experience. How long has the speaker been performing as a speaker. Look for videos of past presentations or at the very least, video clips. Look for comments posted by attendees. Was the speaker informative, funny and entertaining? Would they want him (or her) back again?
Ask yourself, what do I want out of his (or her) presentation? This all depends on the audience. If your group is comprised of intelligent people, make sure your hire a speaker that is very knowledeable on the topic. Nothing bombs faster than a speaker who "wings it" with an audience of intelligent people.
Public speakers can be like advice. When someone gives you free advice, you often get what you paid for. In other words, hiring the least expensive public speaker may result in getting the type of presentation you paid for.
Over my 20 years of public speaking, I have been most loyal to the people that have been loyal to me. If some organization intends to be a repeat customer I always make sure they receive priority. Not only should you do your research before hiring a speaker, but you should also advise him (or her) of your future speaking needs.