Testing +ve and -ve air flow

[beany_bot]

Hi,
I know in theory we can plan our cases around +ve or -ve airflow. (more fans in than out for +ve airflow for example). If there a way of "testing" that we are indeed getting positive pressure in the case? Is there a way people do this? Im setting up what I hope to be a very mildly +ve pressure on my case and would like to test that it is indeed doing it.

 

[beany_bot]

another quick question I will just ask here instead of starting a new thread.

If I plug a 4 pin PWM fan into a 3 pin mobo connection. Will it run at full speed?
and if I plug a 3 pin fan into a 4 pin PWM mobo connection will it run at full speed?

I ask question about pressure because I currently have 2x120 out and 2x120 in. Im planning to replace the in fans to 2x140. Hoping this will give me positive pressure due to the fact they are bigger.