North America has six known species in the Genus Latrodectus. Five of them are present in the continental United States. Most of them are complex and tricky to identify, but others are pronounced. The four most common species are named as shown: Southern Black Widow (Latrodectus Mactans), Northern Black Widow (Latrodectus Variolus), Western Black Widow (Latrodectus Hesperus), and the Mexican species Latrodectus Occidentalis; Latrodectus breaks down taxonomically into approximately 31 recognized species, five of which are found in the United States; four species are native, one species (L. geometricus) was introduced.
Latrodectus hesperus - hourglass primarily complete, but it can vary a lot, from complete hourglass to broken to only one triangle to no hourglass at all, but it is likely that hesperus is actually more than one species (we will see where that lands once the revision of the genus is published). Anterior and posterior halves often proportional, posterior half (next to spinnerets) equal to but never wider than anterior half when hourglass complete; the top of the hourglass is equal to the middle never bigger when the hourglass is complete.
Latrodectus mactans - Hourglass primarily complete. Some papers and sites may mention that the hourglass can be complete, split, partial, or absent, but this is possibly because of the tricky Taxonomic time when North American widows were lumped together in L. mactans and period after where no one was completely sure what they wee looking at. Posterior half (next to spinnerets) always wider than anterior half when complete; top of the hourglass is always larger than the middle never equal or smaller.
Latrodectus variolus - Hourglass often split in middle, with both halves or one half prominent to reduced, or hourglass is partial (anterior or posterior half absent) with either posterior or anterior half prominent or reduced, or hourglass is completely absent. Same problem as mentioned with L. mactans, still unsure to whether they can have complete hourglasses. Adult males show same hourglass variations as adult females.
The rest of the species can be somewhat easily identified
Latrodectus geometricus - Hourglass variation in females, with the exception of color, ranging from red to orange. Male hourglasses do show variation in color and shape, but hourglass unnecessary for aid with ID. Egg sacks are "spikey" unlike all other Latrodectus species. May have variations of the color orange on the abdomen, but never dark red on the longitudinal stripe.
Latrodectus bishopi - Hourglass primarily partial or absent, sometimes split*, rarely complete. Adult males show same hourglass variations as adult females. As the common name suggests (red widow) the legs and head are light red.
Latrodectus Occidentalis - Hourglass primarily complete, but can be split. May be partial or non-present as well, but the certainty on that is unknown. Sides of abdomen have red stripes bordered with white, characteristic of this species.
Factors of identification are pretty much uncertain with varying degrees of reliability. I have compiled a guide with all of the known methods and possible methods of identification located here: https://www.inaturalist.org/guides/18700?view=card
Important Reminder: Even professionals are unsure about what actually are marks for identification as contradicting information is constantly brought in. If multiple marks coincide than it is probably that species, but if you only have one mark and it seems off it could easily be wrong. Use each and every mark in a mind-set of uncertainty until a large amount of the marks lead towards one species.
Important Reminder No. 2: This guide is not an infallible resource. Please remember to check the sources cited and do other background research about Widow spiders!
Sources Cited:
Kaston, B. J. (1970). Comparative biology of American black widow spiders. Transactions of the San Diego Society of Natural History
Genus Latrodectus - Widow Spiders, BugGuide.net
