There are more than 3,800 species of spiders in the United States, including some venomous spiders whose bites can cause serious wounds. Because female spiders can inflict some of the worst bites as well as spread up to 3,000 eggs at a time, it is important to be able to identify spiders by their sex. The distinguishing characteristics of spiders should make identification easy, depending on how much time you have to analyze the spider as well as its size.
Examine the size of the spider. Females are generally larger than male spiders. This step works best if you find more than one spider at a time. Look for the size of the abdomen and the length of the legs.
Inspect the area around the spider's mouth. All spiders have pedipalps, which are appendages all spiders have around their mouths, and differ between males and females. Female pedipalps look like another pair of short legs on the spider. Spiders have eight well defined legs otherwise. On the male, the pedipalps are rounded off and look more like boxing gloves.
Look for distinguishing body marking on the spiders, which vary from males and females. On a black widow spider, for instance, the female has a distinct red hourglass shape, while the male has lighter streaks on its body.