The definition for Meat By-Products is:

"Meat by-products are the non-rendered, clean parts, other than meat,
derived from slaughtered mammals.  It includes, but is not limited to
lungs, spleen, kidneys, brain, livers, blood, bone, partially defatted
low teperature fatty tissue and stomachs and intestines freed of their
contents.  It does not include hair, horns, teeth, and hooves.  It
shall be suitable for use in animal food.

I don't have the definition for chicken or poultry by-product handy.
But IIRC, it reads similiarly.

You have to call the company and ask them for more information.  Some
other important questions to ask:

Do they use a fixed formula?
Do they make their own food or do they outsource it to a third party?
What are their sodium, calcium, phosphorus, and magnesium levels on a
dry matter basis?
How many veternarians work for their company?

Those are just a few questions off the top of my head.  They'll give
you some idea of whether a pet food company is concerned with nutrition
or just making a lot of food.

There's little human market for chicken guts so slaughterhouses sell
them to pet food companies.  In a state of nature a cat will eat the
soft parts of its prey because by instinct they know the "byproducts"
contain the most nutrition.  I've eaten chicken gizzards and livers
myself.

I know that canned dog food comes from the exact same factory as
canned chili.  I'm not naming the factory as that fact might gross out
some of their human customers.

<<How can you tell if a by-product is junk or not? Do companies have to
list (anywhere) the by-products that they use?>>

Ok - let's use chicken as an example.  Here is the AAFCO definition of
chicken byproducts:

"Chicken byproducts consist of the non-rendered, clean parts of the carcass
of slaughtered chicken, such as heads, feet, viscera, free from fecal
content and foreign matter except in such amounts as might occur
unavoidably in good processing practices."

Some byproducts, such as organ meat, cats would eat in the wild.  Other
byproducts, such as feet, cats would not eat.  The problem is that unless
the ingredient label specifies "organ meat only," you have no idea which
part of the chicken is included in the term "byproducts."  The proportions
are constantly changing.  If you buy a bag of cat food that lists chicken
byproducts as an ingredient, they are not necessarily the same byproducts
that are in the next bag of the same food you buy.

If you choose to feed a food that contains byproducts, they should specify
which animal they are from, i.e. chicken, beef, lamb, NOT poultry
byproducts or meat byproducts.  Also, the byproducts should not be the
primary protein source - they should be further down on the list of
ingredients.

I choose not to feed my cats food containing byproducts, unless it is a
consistent ingredient like "liver."

OK, I'm being human or whatever by not thinking of liver as a by-product
because it's something that you find in the supermarket all wrapped up and
ready to eat.  I don't eat it, but plenty of people do.  At least the label
is stating *which* by-product and not just "by-products" which could be
anything.