You know very easily, sometimes dogs don't scratch tick usually if its the first time he will but if this is a requiring problem then you should rub up and down through their hair and you should see a red spot and a little black thing in the skin. You should start by holding a lit match down there for about 3 seconds ( Dog will squirm as the heat hits its skin), but it will be better in the long run though. Then pull the WHOLE thing out including the head! ( Personally I am very bad at doing this so you could take it to the vet for numerous amounts or a tick cleaning shampoo)
