If one finds a wound on their body and does not know when they were wounded, and is not sure whether blood reached their clothes or not, is it necessary for them to change their clothes, or is that not necessary unless they see the blood with their own eyes?

If one finds a wound and is not certain that blood has stained his garment, the default assumption is that it has not. If he sees a lot of blood on his garment, he should wash it until it is removed. If a slight trace remains, it is excused, based on the Prophet's (peace and blessings be upon him) saying: "It suffices you to wash the blood, and its trace will not harm you." If the blood is minor, it is excused.