Honestly that was the hardest thing. Looking at pictures and asking on a facebook group, I was under the impression I had a female (my original). Now that I have the three It is very easy to see the differences.
You won't be able to start to differentiate until around 8" but as I said it may still be difficult. I am unsure what the difference is between the two different locals being Amazonian (yours) and Orinoco (mine), but the tail is longer and thicker at the base on the males when compared to the female. Based on pictures and the location of the cloaca relative to the carapace, and general tail length I was under the impression mine was a girl.
From what I have read on other forums and scouring the internet, males don't get as large as females, they have longer tails, and the triangle on their head stops there (one source). The Plastron also is more concave.
Females get larger/wider than males, their cloaca is closer to the body, the tail isn't as long or as thick, and the triangle continues onto the neck (one source accurate with my trio). The plastron is not as concave. My opinion it is hard to compare without a representative of both sexes present. If you look on the internet you will see pictures.
The problem I found was in the pictures including what others with more experience told me; was that my male had a really short tail. It wasn't until he flashed me a at 12" that I confirmed without a doubt he was a boy. Comparing him with the older larger male and to the female, I can see the difference. It is my assumption that Amazonian males grow longer tails than the Orinoco local does.
I have read that around 12"- 14" their growth slows down significantly and it can take many years to reach the 20" behemoth I purchased. I don't know how accurate that is but it follows what one would expect to be normal growth patterns of sexually mature animals. My original male seems to be following that pattern, he hasn't really grown much in the past two years. He hit 11" at 5 years, two years later he is only 12.5" now. Also Amazonians don't get as large as the Orinoco. They max out somewhere around 18"-20"; where the Orinoco get up to 22"-24".