Well it is really easy actually. The females have a white dot and males dont. The white dot or "egg spot" [like "youlovegnats" said] is sometimes hard to find. You just have to look carefully.
Also (Most of the time) the females have shorter fins. This is not always correct because there have been females with long fins and somtimes there have been females that looked better than the males.
So it is a myth that females are less "flashy" and colorful than the males.
For example here is a female with fairly large fins:
In this picture you cant really see the Ovipositor spot but in the video when the fish goes up you can see the egg spot.
You will be able to just make out the female betta's egg spot under the ventral fins.
This is also a perfect picture that shows the egg spot and how females are a little wider. As "youlovegnats" also was explaining.
This is a male:
Male (PLAKATS)&(HALFMOON PLAKATS) are sometimes confused with females because of their fins being so short.
So the only way to tell for sure that a female is a female and a male is a male is by the egg spot.
Male Plakats (PK's) and male HalfmoonPlakats (HMPK's) USUALLY have really wide and thick looking ventral fins as females USUALLY dont.
I think i covered everything of how to tell the gender of bettas.