How do you know if a vein is damaged ?

Black and blue.: If a vein is damaged from trauma, usually you will see discoloration of the skin either immediately after the injury or several days later. This occurs from blood escaping from the vein and leaking into the skin. Initially this may be dark (black and blue) and over several days as the body absorbs the blood, the color changes to yellow and then resolves.

Damaged veins: There are many types of vein damage (clots, trauma, vein valve dysfunction, inflammation, phlebitis, etc) & each type of damage will show different symptoms. If a vein is large and bulging, discolored, tender, bruised, has red skin overlying it, or is hard when you press on it then it is likely damaged and abnormal. Since there are so many possibilities, i would suggest seeing a vein specialist.

Swollen, painful: If the vein is swollen, red or painful.