Small and secretive, as well as often nocturnal and swift, spiders can be difficult for the layperson to identify. Even so, many people strive to identify eight-legged wanderers encountered in and around their homes --- whether out of general ecological curiosity or deep-seeded willies about venomous or otherwise fearsome-looking beasties. Paying close attention to a spider's colouration and patterning is one of the best ways to identify its species. Many kinds of spiders show white prominently on their bodies.
Study the distribution and nature of the spider's white spots. The white-tailed spider of Australia has, as its name suggests, a dull whitish tip to its abdomen --- the extent of that colour, usually, on its body. By contrast, the six-spotted fishing spider, often encountered in the eastern U.S. along waterways, is frequently intricately patterned with white spots on its legs and white banding on its dorsal side.
Define the spider's general body shape. Jumping spiders are the most diverse family of all spiders, and many show white spots or markings, such as the daring or bold jumping spider. Compared with a web-building, white-marked species like some garden spiders, though, the jumping spider appears compact, its legs proportionately short and stubby, with a relatively huge head. A garden spider, by contrast, has a proportionately small body and thin, elongated legs.
Consider the spider's size. Even though the daring jumping spider, which is white flecked, is one of the larger species in its family, it is under an inch long. The female yellow garden spider, which shows white at the back of its head and sometimes on its abdomen along with striking black-and-yellow patterning, may reach over an inch in length --- and its leg span is notably more. Far larger are certain kinds of white-marked tarantulas, like the Brazilian black-and-white tarantula.
Identify outstanding physical and behavioural features of the spider to help refine your list of possible candidates. The yellow and black markings of the yellow garden spider are a more powerful diagnostic tool than its relatively minor white spotting. The lightning-fast leaping of the daring jumping spider sets it and its relatives apart from any other spiders.
Use a magnifying glass to get a closer look at the spider's spots and other features. This is particularly useful if you can temporarily and humanely corral the animal in a clear-sided jar or cup so that you may examine it from all sides.