After the prior post and mikeselecticstuff's review of a Fluke, and having never used or had one, I got the adjustable Greenlee for reasons I posted on the prior post. It lives up to its expectations and more.
At its lowest sensitivity it will detect and differentiate mains hot vs neutral as pictured. However, its easily calibrated to give quantitative estimates of voltage as proportionate to field strength or to detect mains level voltage at a great distance or behind non-metallic objects than a non-adjustable detector. It runs off a single AAA battery. Its priced in the US at $16, nearly that of no-name adjustable detectors from eBay. Power consumption: 98mA with LED and beep, 2mA quiet. Battery works to 1V. On 1.5V AAA Duracell 1000mAH alkaline this is ~ 10 hours of continuous use.
UNDOCUMENTED FEATURE not in manual: the GT16 makes a solid continuous beep when on and the battery power is too weak to power the device.
The 18 AWG cord is typical of small appliances and the detector alarms when the tip is touched to the hot line. However, when used on a 240V appliance, as expected the tip alarms centimeters away, due to the stronger electrostatic field, despite the heavier gauge wire and its insulation.
At maximum sensitivity the GT16 detect fields from 120VAC below covered objects, in this example, beneath at least 2 pillows or up to 3' away in air. To estimate low voltage AC, as pictured, an HVAC thermostat using 30VAC, the detector at maximum sensitivity in 6 inches from the source whereas with 120V the detector at maximum sensitivity is as far as 3' from the source.
So used thus, one get a bit more information using a variable detector after rough calibration.