There are a few ways to tell if you have a black widow spider. The most obvious way is to look at the markings on the back of the spider. Black widows typically have a red or orange hourglass shape on their backs. Another way to tell if you have a black widow is by looking at the size of the spider. Black widows are usually smaller than other spiders, with adults only growing to be about 1/2 inch in length. Finally, you can also tell if you have a black widow by its web. Black widows typically build webs that are messy and irregular in shape.