Originally Posted by dgwright99

Yes I am sure it's a lie. I am an engineer with deep RF & EMC expertise.
Such equipment could be made in theory (and similar equipment certainly is in use by security agencies, etc), but would be large, heavy and bulky and is most certainly not currently deployed on commercial airliners.

Note that the portable equipment shown in the IEEE article linked by alphaeagle could usefully be used to detect cellphone signals on a plane in flight with reasonable reliability beacuse of the lack of other nearly signal sources. However, it would not give reliable results on the ground, due to proximity of so many other signal sources.

You are also mistaken in claiming that a pilot can "tell" when a cellphone is on. Pilots may, in some cicumstances, observe phenomona that are charactieristic of cellphone intherference among other things but there is no cockpit instrument that could, with any kind of useful accuracy or reliability, distinguish between a cellphone powered on somewhere in the cabin and any one of a large number of other signals present in the envirnment whjen a plane is on the ground.