We sell the Kestrel 4200 which measures CFM.  Yes, this little Kestrel is any HVAC contractor's best friend.

Typically there are used to measure outtake but there is no reason they cannot turn it around to measure in-take.  Here is the process I tell people:

To properly traverse an air duct with your Kestrel 4200 please do the following: 

• Go to the Air Flow (CFM) screen on the Kestrel.
• Hit the Center button to enter the dimensions of the vent:  Shape – Circular; Diameter 18 (or 20 inches) depending on what it was.
• Hit the Center button to exit out.
• Press the Right arrow to get to the Max/Avg screen.  
• When ready to do you traverse, hit the center button to start.
• Traverse the vent in your typical pattern.  The Kestrel will vary for a little but you will see it stabilize after no more than 30 seconds.  
• Press the Center button to stop when traversing is completed.
• The Avg value shown will be an accurate representation of the air flow of that vent.