> the front of the hive covered with bees to check out the incoming bees.

I don't think that any hive would have some many guards, so the bees on the
front of the hive would simply be hot, and the hive needing more
ventilation.

But you have some hard and tangible evidence:

> started doing this 3 weeks ago
> large pile of bees in the front of the hive
> urban area
> I have had pesticide kills before but they never lasted this long.
> hive is loaded with brood, entire frames capped

Well, let's plug what we have together, and see if the narrative works.  The
following is pure armchair detective speculation.

The length of the kill says "ground drench".
The timing of the kill says "ornamentals", as I am guessing you are at the
end of the blooms of just about everything else.

So, it follows that some nice old lady with an outstanding set of flower
beds needs new glasses, and did not notice that the pesticide she bought to
keep her prized flowers pest-free was a concentrate.  She sprayed the
undiluted stuff in a ground-drench, and as each of  the various species of
ornamentals bloom, and with each careful watering, they produce more
poisoned nectar, which kills more foragers, and some (not all) house bees.
It's a big dose, so the foragers die before they can offload, so brood is
not affected.  It is a recent thing, as there was sufficient untainted
nectar to feed all the brood that is now capped.  You may notice that open
brood is a shrinking percentage of brood cells.

You can find this person, just look for the best-looking lawn and garden in
the area, with multiple blooms going on right now.
Or, call the garden club and ask them for a list of members in the zip code
in which you keep bees.

> Having no idea what it is what could be tested?

Sample fresh dead or dying bees and send them down to USDA Gastonia priority
mail.
Not cheap, but a very complete pesticide screening is done.

I don't think they need to do a full battery of tests, as I think you can
narrow it down yourself.
I'd go to the closest Home Despot, Lowest of the Lowes, and local garden
centers, and see what they sell in a concentrate liquid form.
Given the popularity, it is reasonable to guess that you will find several
Imidacloprid formulations, and no other suspects.

Again, I am speculating here, based upon the length of the kill and the
large body count.

Fixing the ventilation problem would free up more bees to be foragers, to
replace those lost to the poisoning, and would also allow better in-hive
housekeeping to keep the hive beetles down.   It would be rare for a
residential misuse of pesticides to kill a hive, here in NYC we have hand
only a handful of such incidents, none fatal for an entire hive.

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