One of the four characteristics of a hazardous waste is “reactivity”. The other three are ignitability, toxicity and corrosivity and these three characteristics are pretty easy to define by measuring the flash point, concentration and pH respectively. Reactivity is a bit harder (less objective) to determine.
The California Department of Toxic Substances Control (DTSC) has a short video clip on how to determine reactivity. In general, if something is going to go “boom”, it is likely to be reactive. Or if the MSDS states that a material is reactive with water or unstable, the waste that is derived from that material is likely to be a reactive waste. Enjoy the video.