Lightweight Chemical Detector (LCD)

LCD (Lightweight Chemical Detector) represents a new generation of
sophisticated yet small and lightweight chemical warfare agent
detectors, which will revolutionise the soldier's personal protection
in the 21st century. Smiths Detection has developed the LCD to act as a
local warning alarm for individual's and small groups of soldiers. LCD
can be hand-held or attached to an individual's uniform.
The LCD
simultaneously detects, identifies and differentiates between types of
CW agents at below attack concentrations. An audible and/or visual
alarm warns the soldier when they need to use Individual Protection
Equipment (IPE). LCD operates continuously and recovers rapidly
throughout the heaviest attack.

This is what Marine Corps Security Forces Batallion (most likely the rest of the USMC as well) uses for detection of chemical agents.

Source: http://www.smithsdetection-sci.com/eng/1385.php