Answer: In my experience, it can be difficult to tell the two apart without the other sex to compare with on site. Here is a photo showing both male and female flowers side by side. Male flowers produce pollen, the female flowers have a central ovary which resembles an unripe berry.
The article accompanying the photo also includes a bit of a discussion about holly reproduction which you might find interesting.
Since the bulk of sales seem to be female hollies, I would expect your plant is probably female -- although that is only an educated guess going by the odds. Good luck with your holly!
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