Just my opionion, but I think we have heard enough about the chickens.  If
there were an economical and fairly easy way to do this, it would already be
done because this is a HUGE industry in this country and the guys with the
bucks would have already figured out how to do it.  It can be done, but not
cheaply or even at a medium cost range.  One would indeed have to rig a
completely separate floor arrangement, suspended by stress gauges or
something similar, get ALL the chickens onto the floor and hope that none of
them decide to go "potty" while they are on it.  Weigh the total and then you
would have to count each and every one of them and find the average weight.
This would cost quite a lot to do and it wouldn't even be all that accurate
as you would probably not get an accurate count and we all know that at least
some of them are going to mess up that floor and that will add to the weight.

There are just too many uncontrollable variables involved to do it within any
reasonable cost frame.

So,  it would appear that hiring some minumum wage folks or just wait until
you take them to market and find out then.