Just have to keep an eye on it as it grows.. the ventrals will seem a bit longer, the dorsal a bit fuller/wider for males. Usually by 2-3 months you will know for sure.. but unsure how old yours is, so can't really help there. Can't go by size, as they all grow different depending upon the water quality, food, etc.

 

Depends on the fish. If it's a veiltail or other large-finned type, you'll know sooner than later, but those fins develop when they're ready. If you provide very clean water and high quality food, you can help ensure proper development.

If it turns out to be a plakat, good luck! I had a whole bunch of babies that were HMPKs and they were a royal pain to sex. Then the anal fins gave it away.

 

Is it easier to sex them based on behavior rather than looks? For instance, can and will both male and female babies flare?

 

You can't sex them according to behavior. Both practically act the same - some aggressive while others more docile. But the males usually show brighter colors. Unfortunately, your baby is white. So you'll have to keep an eye on it until it shows more distinctive signs (fins).

I agree with everyone above - long fins are easier to sex.