turn on a cold tap, bathroom or kitchin, then turn off the stopccok all the way and wait a few seconds. If you have done it correctly the tap will stop running.
hello the last person's answer is not strictly true and could be misleading .the cold water in the bathroom can sometimes be connected to the cold water tank in the loft and it would keep running and would not turn off when you turned off the stop ****.therefore only do the test he suggests by using the downstairs cold water tap at the sink