A female betta who is ready to breed will have a darker color. If she is ready to mate, her body will have light, vertical stripes instead of her usual dark, horizontal stripes.  Her belly may also be slightly enlarged if she is gravid with eggs. Her ovipositor will also be more visible. It shows up as a white speck around her ventral fins. You can also tell by her behavior toward the male when put together. If she's ready to mate, she'll "flirt" with him by flaring her fins and wiggling her body.