Inside water stopcock doesn't work. Want to check if the right one

[pulak25]

Hi, I have a stopcock in my closet at my flat, but despite me turning it clockwise all the way, this doesn't seem to be stopping the flow of water.

Is there anyway I can check if that truly is the stopcock and if not, is there a way I can find the actual one?

I just want to stop the water flow temporarily so I can change my shower valve