Necropsy is to animals and other creatures that autopsy is to people. That is, an investigation into the details of death.
When we are asked to inspect the cause of a hive die-out by a client or on one of our own, that is doing a necropsy.
There are a vast number of ways bees can die, individually and as a colony. My approach to necropsy is similar to how I go about analyzing a computer/networking problem. I start by observation and with the most basic and common problems first.
Any investigation has to begin with observation and evidence. If you're unsure of what exactly those might be in regards to this task, I am affixing a flowchart here to have something to start with. It is NOT complete not is it entirely specific. The purpose is to get an investigation started and moving in the most likely accurate direction.
Always start with the most obvious possibility. Just like diagnosis of a computer issue, the reality of beekeeping is to start with the user, in this case, the beekeeper.
Beekeeper error is usually the most common problem. The beekeeper added something, removed something, changed something in the hive that led directly or indirectly to the problem. Beekeepers are loathe to admit beekeeper error. They hate to think it is their fault. Try not to be accusatory but yet, don't allow their feelings to mislead you in the analysis.
Look at the hive itself, the Apiary grounds, the location and condition of nearby structures and objects like trees, bird houses, etc… If an environment has a predisposition to host pests and predators or to become contaminated such as a food or water source, these should be considered early on as the next most likely possibilities.
Never, in these times, should we discount the likelihood of sickness in the hive. Everything from dysentery to Foulbrood are possible. Often they may be connected to one of the other categories such as beekeeper error and environmental conditions.
The quality and quantity of area forage sources need to be considered especially in terms of availability as well as timing and diversity.