It sounds like you want to measure the I-V curve of your solar panel.
The solar panel will operate at a point on it's I-V curve depending on the load it is attached to. To get an idea of what a solar panel I-V curve basically looks like you can see **broken link removed**.
The basic idea is to measure the current and voltage for various loads. So, you set a load so to "force" a current and then you measure the voltage. Then you repeat for a range and currents and produce a graph. (alternativly, you can force the voltage and measure the currents).
There is equipment that can do this. But, for a cheap and laborious way to do it ... you can use a poteniometer with a voltmeter and an ammeter. Hook the pot up as a load to the solar panel. Measure the voltage across the pot with one meter and measure the current through the pot with another meter. Tune the pot to a range of voltages and plot the corresponding currents (or vice versa). Then you can have a pretty good idea of what your solar panel can do.