The three poisonous spiders in the US have some identifying markers. Black widows are completely shiny black with a recognizable red hourglass shape on their backs. These spiders are rarely found indoors except for garages, basements, and attics, so you are more likely to see one in the yard or nature. However, they do like to hide in and under wood, which means it's possible to accidentally run into one around your home.
Brown recluses are, as indicated by their name, brown and about the size of a quarter. They have thin legs and a thin body. A brown recluse bite can be painless at the time, but will develop inflammation and a burning sensation over a few hours. The bite will usually heal in 3 to 5 days, although some people do experience more severe reactions and require medical attention, especially children and older adults.
Hobo spiders are around an inch to two inches in diameter with larger abdomens and thin legs. They are a light to medium brown, and have a yellow pattern on their backs. Because they are part of the funnel-web family of spiders, hobo spiders make webs that are shaped like a tube or funnel. A hobo spider bite can cause some short term pain and redness around the site.