You could measure it with a multimeter obviously, or take reading with analogRead and calculate it based on the value of the other resistor in your voltage divider.
Why do you care though? You're going to get analogRead data from the divider somewhere in the 0 to 1023 range. All you need to do is find out by experiment what values correspond to the light levels you're interested in and then use them as thresholds to decide when to move your sun tracker. The actual value of the resistance needn't matter as long as you get a discernible difference in readings when sun conditions change.