Thank you for reaching out with this beekeeping question and taking time to learn about colony losses. Unless a colony ran out of honey and starved, we attribute most winter losses to varroa mites and the viruses they spread. Honey bee colonies can survey cold winters if they have sufficient honey and are healthy. Unfortunately, the viruses that are spread by varroa mites and bees within the colony are often at high levels in the fall when winter bees are developing. Winter bees that are exposed to high virus loads can have difficult time surviving the winter. Honey bees carrying high virus loads may appear healthy.

You can look for varroa mite frass (excrement) on brood frames on the ceilings/top walls of cells. It looks like a little dot of white out. We find varroa mite frass in cells after a varroa mite has reproduced in the cell. Honey bees frequently clean cells, so finding varroa mite frass is usually an indication of high varroa mite loads. I attached a photo example of lots of varroa mite frass (photo credit to Dan Wyns).

Michigan State University does not offer diagnostic services for honey bees. You can send samples to the USDA ARS Bee Research Laboratory in Beltsville, Maryland. Note that this diagnostic service is free of charge (other than payment of shipping samples to the lab), but it can take a while to receive results.

Here is a list beekeeping resources. Thanks for taking time to learn about beekeeping!