well, thought this might be helpful to some people. First off, you may want to study betta anatomy first...
Males typically have longer tails, and a longer anal fin. Males also blow bubble nests, and flare. Females have an "egg spot" on their bellies right in front of their anal fin. If you don't know what gender your betta is, try the following tips:
Place a mirror in front of your betta. If your betta starts to open his fins up, and swim towards the mirror, almost trying to attack it... you are probably in the presence of a male, hm hmm. What the probable male is doing, is called displaying, or FLARING. If your betta doesnt react, don't worry! your betta could still be male, just an older male....
Are there bubbles at the top of your betta's home? If so, you are looking at what is called a bubble nest. Your betta is almost definitely male, since it is EXTREMELY EXTREMELY EXTREMELY RARE.... like so rare that betta hobbyists for 50 years havent encountered it... for females to blow bubble nests. But don't worry, if there are no bubbles your betta could still be a [lethargic, unhealthy] male. But more likely unhealthy, or very old, since lethargic bettas blow bubble nests often.
Is there a tiny white spot on your betta's belly? One that is right in front of the anal fin? If so, you are either in front of a very pathetic juvenile male, or a female :). Juvenile males look nearly identical to females as far as finnage goes, but they can blow bubble nests and flare.
Sound confusing? I bet. Here are some tips:
Females don't have large fins. If you have an old betta who won't flare at his reflection and blow bubble nests... and has too big of fins to check for an egg spot - fear not! Old bettas are notorious for BIGBIG fins. Most pet store bettas are old. Because ignorant people wont even look at a betta who doesnt have huge fins. Little do they know, Their betta is viewing his stay at their home as a nursing home... the last stop in his journey of life before he kicks the bucket.
Females are solemnly sold at pet stores, so if you got your betta at a pet store it is probably male. Ho-hum. It is also probably old.
Males are generally more appealing and attractive than females.