It might be quite uncool to check an emergency shower!
The spray head is located unreachable and if the shower is switched on lots of water is release! So isn't it best to switch it on only a very little bit and only for a very short period of time if it has to be checked? After all you do not like to become wet, don't you?
Unfortunately the pipe has to be flushed with plenty of water and it has to be checked that the
water flow is at least 30 l per minute.
Prepare a usual waste water pipe of suitable length with an angle piece at one side. Put the other end into a bucket with a graduation. Have a stopwatch in one hand. Then it is very easy to check the flow without getting wet.