Spreading has its own signs, separate from the signs that a wound is infected at all, and they are worth knowing as a list of their own.
With cellulitis, the central one is skin redness or inflammation that gets bigger as the infection spreads. Another is a skin sore or rash that starts suddenly and grows quickly in the first 24 hours.
Spread is measured rather than guessed. Your provider may mark the edges of the redness with a pen, to see if the redness goes past the marked border over the next several days.
If you are already being treated, report these new symptoms right away: persistent fever, drowsiness, lethargy, blistering over the cellulitis, or red streaks that spread.
Drowsiness and lethargy are on that list and are easy to put down to being unwell generally. They still count.
Some situations may mean hospital care: being very sick, for example with a very high temperature, low blood pressure, or nausea and vomiting that does not go away; being on antibiotics while the infection gets worse, spreading beyond the original pen marking; a weakened immune system; infection around the eyes; or needing antibiotics through a vein.
Untreated or unsuccessfully treated cellulitis can lead to: blood infection (sepsis); bone infection; inflammation of the lymph vessels; inflammation of the heart; infection of the membranes covering the brain and spinal cord; shock; and tissue death.
That list is the reason spreading is treated as its own question rather than a matter of degree. Wound Wizard has a separate article on sepsis, which is the one of those complications most often asked about.