Most of the major ad networks let you assign a code to each of your ads (or each type of ad), so that you can keep track of what's getting clicked.
But don't expect to see any revelations. I've had periods of several months where almost nobody has clicked a banner, but thousands of people clicked on the text ads. Then, without any action on my part, the situation got reversed. I've also seen huge variations between different sites.
I suppose it all comes down to which advertisers are active in a given market at any given time, how much they are willing to bid for ads, and whether they prefer text, graphics or whatever.