I would first interview the agent or agents and ask them certain questions that you might have regarding their experiance. I have found that the more you ask them about real estate you will get an idea of what they truely have to offer. The other thing is that there are so many agents out there, just find one that you feel the most comfortable with.

Just ask them! Usuallly an agent will (sometimes) tell you the truth. I would just Google their name and see what comes up.

I'd rather answer this by saying first look for Realtors, then use only full time agents then weed them down to your locality and the surrounding areas then ask for their experience and credentials( any designations like CRS, GRI, etc) are they Salespersons/Associates or Brokers or Associate Brokers. Interview them for compatibility and make your decision.

Go to the company website and look up the agents profile page.This usually gives you an excellent reference for what you are looking for.You can also Google the persons name,and you will see exactly who you are dealing with. The most successful agents, the more his or her name will appear on your Google search. For instance, if the person you are looking for comes up on Google numerous times,that person advertises often in top real estae publications,the more their name appears,the more successful that person is.

Edith
Your profile says that you are a Home Buyer.
If I were a home buyer I would interview a couple of Realtors (there is a difference between a REALTOR and a real estate agent. Realtors have to agree to abide by a code of ethics.).
Interview Realtors from different companies (meaning not three associates from the same firm). I (being unashamedly biased) suggest the local Keller Williams office as a place to start.

The reason I say interview them is that buying or selling a home is a complex transaction. You will be dealing with the professional you select for quite a while, so you need someone with whom you can relate.

I think the number of transactions is important, but for more important is someone that will listen to your needs, not sell you on something.
Someone that demonstrates that they pay attention to detail, in particular to what you say (so they ask questions, listen, and take notes).
Someone that has a strategy to help you. Driving around showing people homes is not a strategy. Helping you determine your value and priorities and outlining a process is a strategy.

Ask them what sorts of problems they have encountered and how they have dealt with them (can they problem solve). Ask them how many properties they will show you (hint. the answer should be one that shows you they do expect you to be serious and make a decision, once they are showing you homes that meet your needs. )
Lastly I do not see a relationship between the number a transactions that a Realtor has, and their degree of professionalism. I know Realtors with 30 years experience who do not appear to have moved along with the times. I know Realtors with six months of experience that have learned from not only their own experiences, but lean on the experiences of others, and are like a spong. They are not afraid to say "I don't know the answer to that question. Let's find out.".

A Realtor that knows it all may know less than you think. At Keller Williams our philosophy is that we are all "liftetime learners".

I hope this is helpful.

Look on their company website. Most agents give out their bios, which list their homes sold. Make sure you're seeing homes sold and not homes LISTED because there are power listers who overprice their listings badly and they all expire.

Edith,
Like the other two agents said, just ask. I provide my resume and background information to any Buyers or Sellers that ask. Plus I post it on my website. Another thing that is important, is that you interview the agent to make sure they have the personality that complements yours. You will be working side-by-side with this person sometimes for many months, and it is important that you can get along.
Good luck!

Ask!

But may I add IMHO I believe it is the overall ratio and not the total number that should be most important to you. For example, Agent # 1 has only a year in the business, has listed 2 homes per month for a total of 24, and all but 1 sold within the listing period. Agent #2, 6 years in the business, over the last year has listed 40 homes, but 10 of those expired without sales. Agent #1 has less experience but a better batting average.

I don't know if you are looking for an agent currently, but all I am saying is weigh your options carefully. Good luck!

It's simple, ask them. Most quality agents track their sales. For example, I could show you a list of every house I've sold over my 12 years in the business.