Most open sores in this position are venous ulcers. Most venous ulcers occur on the leg, above the ankle. So the signs of infection to watch for are the ones for venous ulcers.
Contact your provider if there are any signs of infection, such as redness, increased warmth, or swelling around the wound; more drainage than before or drainage that is yellowish or cloudy; bleeding; odor; fever or chills; or increased pain.
There is a complication on a venous leg that does not apply elsewhere, and it is worth knowing before you use that list. Several of those signs may already be part of your normal. With a venous ulcer, the surrounding skin may be shiny, tight, warm, or hot, and discolored. Early signs of venous insufficiency include leg swelling, heaviness, and cramping, and dark red, purple, brown, hardened skin (this is a sign that blood is pooling). A warm, swollen, discolored lower leg can be the condition rather than an infection.
That does not make those signs useless. It means you watch for change rather than presence. Warmth that is greater than it was, swelling beyond your usual, discoloration in a new place or a new shade — those are the signs that tell you something on a leg like this.
Two signs are tied directly to infection, and neither is part of the baseline. If the sore becomes infected, it may have a bad odor and pus may drain from the wound. Drainage works the same way. A venous ulcer that leaks is not news, but more drainage than before, or drainage that is yellowish or cloudy, is.