My male was never overtly aggressive. Not even when he reached maturity. The only fish he was particularly aggressive to were blue coloured females.
I had long suspected my young 'female' was in fact a male. However, it wasn't until he matured and his form changed that I could confirm it.
Here is what he looked like when he was fully mature. He had a less rounded body than my females. Not only that, but his ventrals were thicker, his beard more pronounced and his anal fin more sharply angled.
This was what he looked like when he was still young (he is the blue and white marble)