Air Condition Coil

The outlet pressure is not very relevant (only a bit above zero); flow rate is more important, and should be proportionate to the internal volume of the unit being brazed. Usually a rotameter is used to check this.

I don't know how the pros do it, but if I had to know when nitrogen was coming out of a pipe, I would put a lit cigarette in the exit flow. Nitrogen doesn't support combustion, so the cig would go out when it arrived.

Just make sure that the welding gas is not in the pipe, or the cig will ignite that!

Just feel the flow at the back of your palm is sufficient for the purpose. No any measurement of flow or pressure is necessary. But unless it is confirmed that nitrogen is coming out from other end brazing should not be started. It is prudent to wait for a minute so that inside air is driven out by nitrogen.