Besides the trademark drool and crankiness, try the gum-massage test. (Babies are more likely to accept a finger probing into their mouth than to allow a look.) Run your finger along the front edges of the gums, and you will feel swollen ridges of preteething gums.
It is sometimes difficult to tell if a baby is teething because the amount of teething discomfort varies considerably among babies. Some are steady, once-a-month teethers; many teethe in bursts and pauses, where suddenly baby has a miserable week and you feel four swollen ridges along the gum line. Expect the most discomfort when many teeth come through at once.
Some babies experience exquisite pain and swelling during molar teething. If allowed to look, you may notice a mound of swollen tissue around a budding tooth. Don’t be alarmed if you notice a mushy blue blister above an erupting tooth. This is actually a collection of blood beneath the superficial layer of gum tissue. These painful swellings are best treated by cool compresses (for example, popsicles), which soothe the swelling.