Usually when test results are potentially affected by many factors, standard test conditions are defined. When tests are performed under these standard conditions, the results can be compared between products.
Since these standard conditions rarely exists in nature, testing under standard conditions is not possible. Instead, a test programme is developed that tests under a number of non standard conditions to identify the effect of individual factors and correct for that. In addition, components of the system under test can often be tested in an isolated controlled environment. In this way, standard conditions can be achieved for individual components and the effect of deviations for the standard conditions can be determined as well.
For the example of a solar aircraft, the efficiency of the solar panels could be determined independent of the aircraft. The same holds for the battery, the engine, the propeller and other components that the propulsion system may depend on. Not only the performance under standard conditions can be determined, but also under non-standard conditions.
The power requirements of the aircraft under a range of conditions can be determined by test flights. This could involve horizontal cruise flight segments at various speeds, altitudes and temperatures, and also gliding tests.
The maximum achievable flight time under standard and non-standard conditions can then be calculated from the results of all the acquired test results, without ever having flown under these standard conditions.
For validation of the methodology used, a test flight can be done do determine the actual flight time under non-standard conditions. When the results of the test flight agree with the calculated flight time under the same non-standard conditions, one can assume that the methodology is correct.
For you own RC plane such a test programme is probably out of scope. A simple way to gather some data is to perform two flights with the same aircraft. Both time start with a full charge (I assume there is a battery involved). Test how long the flight last with the solar panels. For the second flight, disconnect the panels and see how long the flight last then. You now know how much the solar panels contribute in terms of power production. Then fly with the non-solar plane and see how long that lasts. It will probably last longer than the solar plane with the panels disconnected because it doesn't have to carry the extra weight of the panels. This way you learn what the addition of solar panels costed in terms of flight efficiency.