If someone is naturally polyamorous, will it be shown in their birth chart? Are there other ways of knowing whether a person is naturally polyamorous or not?
Yes, the girl I love has a tendency to cheat on her partners and fall in love with multiple people at the same time and in fact it showed in her birth chart.
But considering the partners she finds, I wouldn't "blame" her. Always consider the chart as a starting point to understand how you are and the areas of life to develop.
For example, that girl cheats, but she has never even been in a relationship where she can feel loved (and this was also evident from her birth chart).