How fo uou know when you have cellulitus?

Red, pain. Skin infection-- redness, tender, may extend without treatment, may be associated with swelling, fever.
Pain, redness. U will notice redness, pain, swelling and area would feel warmer than other areas.
Swelling, pain, etc. Usually cellulitis presents as increased pain in the area, redness and swelling. It usually has a "sore' as the starting point and spreading redness and swelling.