The abdomen is the third region of the insect body. Like all arthropods, insects have segmented bodies. The number of abdominal segments can vary between insect orders. The abdomen may also have appendages that are clues to the identity of the mystery insect.
The number of abdominal segments varies from six to eleven. For example, silverfish usually have eleven segments, while springtails only have six. If they are visible, try counting the segments.
Your mystery insect may have an obvious “tail” at the end of the abdomen, or what appears to be a set of pincers. These structures are touch organs that help the insect feel, and are called cerci. Earwigs have modified cerci that function as forceps. Three-pronged bristletails are named for their three cerci.
Note the size and shape of the abdomen as well. Is the abdomen long and slender (like in mayflies)? Does it look swollen compared to the thorax? Some identification keys use these characteristics as well as the others you have already observed.
Photo credits: (top left) Scott Bauer, USDA Agricultural Research Service, Bugwood.org; (top right) Flickr user FastPhive; (bottom right) Susan Ellis, Bugwood.org; (bottom left) Susan Ellis, Bugwood.org