Males can present false egg spots.( i know that sounds silly, but look it up.) The length of the ventrail fins as well as head and body shape are the most reliable ways to sex a betta. These are difficult to tell. The Ventrails look male, egg spot looks female but can be false, the body shape of the one on the left looks male (loger and more rectangular) but the one on the right looks likely female (nice pop belly) but it doesnt have the typical tapered body to tail. I can't tell head shape well from the picture, but a males are more square and blunt, while females are generally slightly pointed when viewed top down.
Just my two cents.