There is no short answer to this question but to start you definitely should know the market in the area you are looking and be able to spot a good deal when you see one and you should not be searching for anonymous 3rd parties.
Try talking to a few to start off, discuss what your looking for and see if they have they can or will assist you. Ask if they work with any other investors, and through your dialog see if they ask a lot of questions and then shut up to hear your answers or not. That is a good way, the best agents do more listening than talking but when they talk should be asking you a lot of questions, not idol chit chat, bragging or other BS! The best agents are the busiest ones.
If you're an investor you should have experience with agents, if you're a new investor you should be bright enough to detect if the agent is knowledgeable or even easier has a lack thereof.
Real Estate has many fields and many specialties, what area do you need the realtor to be an expert in?
Commercial, Industrial, New Construction, Land, Condos, Co-Ops, Resale Residential, HUD properties, Foreclosures, Short Sales/Pre-Foreclosures, tare downs, needing major rehab or rehabs just needing updates, single family, multi-family or...?
There are many kinds of "investors", you said you're a rehab guy. Are you looking for realistic deals or trying to get homes for 10 cents on the dollar that just need updates?
There are many types of investors and the realtor will need to find out which one you are: Builder, Rehab, Flipper, Landlord?
Are you buying all cash or will you need a mortgage if so do you have the necessary cash to put down and are you willing to show proof or just act like a hot shot saying money is not an issue? Money is always an issue, when I hear that line, I ask so if I found you a good rehab job that will payback big time but the property will cost a million dollars, you'll be okay with that kind of purchase. That is when you get some insight how much money really is an issue. Investors need special financing, not 3.5% down FHA loans!
Experienced agents know "investors" are not loyal to agents at all and much like agents there are a million investors out there, many of them of the "wanna be" variety that just watch too many late night infomercials and attend way too many investor conferences without really having the knowledge, resources or cash so if you find a good agent, you'll need to show a little interest and loyalty to work with them.
The same way the realtor bashers portray agents is the same way Realtors look at a lot of "investors" because usually the agent makes little out of these deals if the investor is loyal and actually purchases thru the agent, usually won't use the agent to resell the property and will expect the expert agent to waste a lot of their time researching hot deals and keeping you in mind and bringing the deal to you instead of one of their loyal clients or even buying the good deal investment themselves!
Those are just a few things to keep in mind. As a businessman you'll need to think from the agents point of view as they will be your deal hunter.
Investors typically think nothing of wasting a lot of agents valuable time so they may blow you off if you don't want to share any info and just want the agent to search constantly for properties for you and email or fax them to you! NOT a good start. You can get many new agents to jump at your feet for that. You have to present yourself well to the agent as when introducing yourself as an investor because investors aren't shy telling you want they WANT but don't want to reciprocate with a little info which is the quick way to get blown off. Experienced agents have good BS sniffers and not afraid to call you on it or walk away from people they think will just waste their time.
The only true way to find an expert is experience, relationship building and word of mouth, you may have to go through a few bad apples before finding a gem or one that will want to work with you. Investors and part time agents usually don't make a good match however I have seen a few of these relationships work successfully where the investor both buys and resells through the same agent and continues to use them instead of bouncing from agent to agent.
Listening to posters like Realestate#1 will get you nowhere and that is obvious. These types of people here on Trula always advise not to use Realtors and just bash us at every turn usually having more than one profile to do it with to. http://www.trulia.com/blog/victor_kaminski/2009/04/fake_trul