The main way to tell the difference between these two conditions is their appearance.
Thread veins are the less aggressive looking of the two and present as thin red or purple lines under the surface of the skin and are usually associated with a lack of exercise, hormonal changes or standing for long periods of time. They mainly appear on the legs although can develop on the face as well but they are relatively pain-free.
Varicose veins on the other hand are more pronounced and present as enlarged or swollen veins that are usually very dark purple or blue in colour. They mainly develop on the legs and feet, with some extreme cases experiencing bulging veins that appear lumpy or twisted. They occur when blood collects in one area of the vein and often need to be corrected by surgery.