> To refine Charlie's post, pull out the brood frame 
> upon which the bees last clustered.  Look for an 
> area with a scattering of dead sealed brood or
> emerging bees.

While the above is accurate advice for someone looking at a deadout in fall
or early winter, it is not applicable to a mid-winter or spring deadout.  

The reason why is that the final mid-winter or early spring clustering
location of the bees before they died is not going to be where the last
"fall brood" emerged.  The actual probability of that being the case is so
close to zero, that one would be well advised to look everywhere EXCEPT
where the dead bees are found clustered.  

The reason here is simple - bees eat stores, and move into the comb space
created by consuming the stores.  This tends to be an "upward" movement.
The bees will never move away from stores in winter, so where the bees are
found dead in their forlorn tiny final cluster is highly likely to be above
where the brood last hatched out.   The longer the colony survived winter,
the further away from the fall brood area the dead bees will be.

That said, varroa is rarely the cause of a "failure to overwinter".
Colonies crash from varroa most often well before Christmas in areas where
there is a significant winter cold period, at least in my own experience.  A
minor varroa problem that remained untreated certainly can result in a
smaller colony that gradually loses population over winter, and drops below
the critical mass of bees to survive a cold snap, but in this case one will
not find clear evidence of varroa, as the varroa did their damage back in
fall, and the colony simply took several months to spiral down to a doomed
number of bees. 

Simple starvation and simple "lack of critical mass" are the most common
failure modes for colonies in places that have a winter worth talking about.
I have a hard time convincing people in the NYC area to NOT wrap their hives
with insulating materials, but to instead, wrap only the space under the
hive to create a dead-air space and block drafts through the bottom.  Those
who wrap their hives, or worse yet, try the Styrofoam hives, see much higher
rates of stores consumption, as the bees stay active longer when they are
insulated, and active bees eat much more than clustering bees.

If the hive died in early spring and was raising brood, this evidence alone
tends to discount "varroa" as the proximate cause of the death of the hive.
So, if you do find dead bees and brood on the same frame in spring, and the
hive was alive at Christmas, it is almost certain that the bees would have
survived to spring with enough stores and a larger clustering population.  

Not that it helps now, but counting varroa is best done with a sticky board
while the hive is still alive.  Over winter, sticky boards can also "map"
the size and approximate position of the cluster from the debris that fall
as well as allow one to screen for varroa that survived fall treatment or
came back to the hive with bees that robbed out weak colonies or fall
deadouts.

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