The best way is is to monitor your bowel movements on a daily basic. watch to see if your bowel movement has becomes irregular.If there is any change of colour,such as becoming more dark,it could be a sing of stomach ulcer,if you see fresh blood then it could be from the colon in which growths,which may or may not be cancerous ,or they may be hemorrhoids,which are treatable. in either case you should see a doctor immediately for testing,and find out the cause of the change