I used to carry a regular electric tester. Then a couple years ago I upgraded to a "Fieldpiece" brand. Not only is it an electrical tester but it is built to accept many different attachment heads for different purposes. I have the AMP clamp, the carbon monoxide detector head and for this thread the CFM meter head.
When I'm especially suspicious of a system, I'll pull out the head and measure the CFM at various registers and grilles. It really comes in handy to prove a point. Also cuts the BS from the hvac guy way down.
Fieldpiece Instruments - Hand Held Instruments For HVAC/R Installation And Maintenence
I've had mine for a couple years and they work great, even after many falls.
Fluke and some of the other companies are also making this type.