How to measure the maximum power of a solar panel
Assuming the current/voltage relationship is linear (it's not, but this gives you a crude lower bound), you could measure the short-circuit current and the open-cell voltage and do 1/4 * I * V to obtain the maximum theoretical power given a worst-case 0.25 fill factor.  However a more reasonable value might be obtained by using a different factor


  Typical fill factors range from 0.5 to 0.82. (source)


Because the I-V curve is non-linear you either need to find the parameters of your cell and apply equations (such as found here), or simply do it numerically by taking measurements at various I/V and finding the largest I*V coordinate.