yes... with the proper equipment and proper procedure, but if you hook it up its easier.
its easy to test the output voltage of the panel by itself, but load testing it requires more equipmet than you probably have. the following procedure will kind of do both to some degree. probably well enough for your needs.
you didnt say how many watt the panel is, but if it is hooked up you can use a standard multimeter to test it.
check the battery voltage before you connect the solar panel.. write the reading down.
then hook up the panel and let it charge for 15 minutes or more, then check the battery voltage again... it should be slightly higher if the panel is working.
im not sure how important it is, but it is highly recommend that the panel is not connected under load... my interpretation of that statement means that the battery needs to be at rest and not discharging/working, and you should have a blanket over the panel. then plug the panel in... and then remove the blanket from it.
others may have a more sophisticated and refined test, with actual numbers to go by
