The back of the head is one of the bony spots where pressure sores form, along with the buttocks, elbows, hips, heels, ankles, shoulders and back. So for someone who spends long periods lying down, a wound on the scalp may well be a pressure sore, and those are the signs to use.
Contact your provider right away if you develop blisters, an open sore, or any signs of infection, such as a foul odor from the sore, pus coming out of the sore, redness and tenderness around the sore, skin close to the sore that is warm or swollen, or fever.
The hard part here is not reaching the wound but seeing it. Hair covers the scalp whether or not anyone is looking, so checking your skin for pressure sores every day means parting the hair and looking at the skin, not glancing at the head. If someone else is doing the checking, pass that on to them.
Position is the other half. If you are in bed, you should move or be moved about every 2 hours. Pressure can be reduced with pillows, foam cushions, booties, or mattress pads, chosen with your provider rather than by guesswork.
A scalp wound may of course be a cut, a graze or a surgical wound instead. If so, the general signs apply: redness with increased pain, yellow or green pus, or excessive clear fluid around the injury; black edges, which signal dead tissue; fever of 100°F (37.7°C) or higher for more than 4 hours; pain that will not go away even after pain medicine; and bleeding at the injury site that will not stop after 10 minutes of direct pressure.
One rule is worth repeating here, because a scalp wound tends to attract whatever is in the bathroom cabinet: do not use hydrogen peroxide or iodine cleansers unless directed by your provider, as they can damage the skin.