Good question! Fire severity is a great way of attempting to quantify the variable impacts of fire. From a remote sensing perspective (typically satellite), there are multiple techniques that have been developed to identify and map patterns of post-fire burn severity. Many of these techniques use multispectral satellite sensors. The normalized burn ratio (NBR) and its variations (dNBR) come to mind. There is even a national (US) archive of burn severity maps available for free online from the Monitoring Trends in Burn Severity (MTBS) project led by the USGS (https://www.mtbs.gov/). For assessing burn severity from the ground, you might consider looking into a technique called the Composite Burn Index (CBI). Link for more info here: https://burnseverity.cr.usgs.gov/products/cbi.