Some law enforcement has access to handheld/portable versions of chemical identification tools that have been historically restricted to laboratory use. Customs agents frequently use FTIR and Raman spectrometers but some have even portable mass spectrometers that they can use (relatively newer tools).

FTIR/Raman works by using light (infrared and visible respectively) to identify chemicals in a sample. Briefly, this works by looking at the 'color intensity' of the light that went in and looking at how much is missing in the light that goes through the sample. The color of light absorbed by a chemical depends on that chemical's structure. Most chemicals (when pure) can be uniquely identified by their FTIR or Raman spectrum (the spectrum is a graphical/numerical description of the colors that were absorbed and by how much from the sample).

How accurate it is depends on the sample. If the sample is pure, the technique is highly accurate. If its a mixture, it's going to depend on what the components are, how much of each there is, and what you're trying to look for. FTIR/Raman often struggle to specifically identify inorganics like minerals, but are very good at identifying organic compounds like cocaine and methamphetamine. FTIR/Raman also struggles with low concentration formulations. For example, if cocaine is present in a sample at less than 5% with other material such as lactose composing the majority of the sample, it may be difficult to identify by FTIR/Raman.

Often, law enforcement will use a variety of techniques to identify material to overcome the shortcomings of any one technique.

For the most part, the hand-held versions of these devices are designed to operate automatically - interpreting the data for the user and identifying it as accurately as it can. The device usually gives some indication of confidence or whether or not it's a mixture. It's important to note that the identifications from these devices are almost exclusively used as a presumptive analysis, meaning that confirmatory identification from a lab is necessary for any type of criminal charge. This means they can use it as presumptive cause to take some action like seizure or arrest.