Many Western tropical astrologers consider the Moon to be void when it will not make another Ptolemic aspect (conjunction, sextile, square, trine, opposition) to any planet from the sign it currently resides in. And most modern tropical astrologers extend this to include the outer planets, those not visible to the naked eye: Uranus, Neptune, Pluto.
Traditional astrologers however do not consider the Moon void if it is in an applying aspect to a planet within 13 degrees (when the body is in orb to the Moon). And the Hellenistic doctrine has us extend this to 30 degrees! Traditional astrologers, and some modern astrologers, also do not take the outer planets into consideration when examining a void of course Moon (though many use outer planets connecting to the Moon in character delineation in a natal chart or for other purposes).
If you're a modern tropical astrologer, I encourage you to experiment with your practice. Utilize the traditional rules. Try using only the seven visible 'planets' for a while - Sun, Moon, Mercury, Venus, Mars, Jupiter, Saturn, and add the outer bodies as an afterthought. You might find working this way enhances and simplifies your interpretations and conclusions, cutting through to the heart of matter!
When the Moon is void, or "running in the emptiness", 'running through a vacuum', 'feral', 'desart', 'wild' or 'isolated' (Skyscript), a pretty rare thing by Hellenistic standards, it is best not to initiate new projects. It is however a great time to meditate, sit in silence, or go for walk. Take in the spaciousness and emptiness of a void-of-course Moon - it is an optimal time for retreat and contemplation.