Interesting question. It apparently is known within the trade because the highest grossing agent in my area advertises her success widely. As part of her sales brochure, she sends out a sheet with a list of the top 25 agents by sales with the number and amount of sales made in the past year.

I work in a RE office and I can look up any agent in our MLS to see what he/she has sold in the last 6 mos., year, etc. I don't think that info is available to the public though. You can ask each potential agent how much they have sold.

Another thought - the agents that have sold the most in my office are selling foreclosures at rock bottom prices.

If you do find out, might I suggest you then find out from the selling and the purchasing parties how pleased they were with the transactions. Some of them I know are only interested in selling homes in certain dollar categories. That's fine if your home just happens to fall in it. There are a lot of very fine, hard working realtors out there who work their tushes off making sales on a greater number of modest houses whose sales figures look pale in comparison. But like Larry says, they get 'er done.

There is also a very fine line between being a go-getter and being aggressive to the point you burn a swath behind you. Agents like that burn bright until their reputation starts to precede them.

Here is another suggestion. Go on to Realtor.com, or your local site, and read different listings. Look for well written listings with more than 10 pictures. Look for customized copy on the listings. Sales are important, but it doesn't mean much if the listing can't catch someone's eye. I suspect the best realtors meet all these criterium.
Jo