Things you might not have to worry about
With a lot of information overload, it can sometimes be tricky to pick out the "really dangerous stuff ferrets should never have in their food" from the "you might want to avoid this if the quantity is really excessive".
Here's a few things that newbies sometimes get caught up in that you might not need to panic about as much as you expected:
"There is a large number of fruits and vegetables in this food"
The number isn't important, it's the amount that counts.
The first 5 or 6 ingredients (or some say everything up to the first listed fat or oil ingredient) make up most of the food, and the stuff near the end of the ingredients list is just there in trace amounts.
If there are lots of vegetables early on in the list (ie. 4 out of the top 6 and/or if a vegetable is the 1st or 2nd ingredient), it's usually bad. That's a higher carb content than a ferret should have. That's often "filler".
If there's 1 or 2 further down in the first 6-ish ingredients (ie. the 4th and 6th ingredients are vegetables), that's completely normal. In a perfect world it wouldn't have to be that way, but dry kibble usually physically can't be made without at least some vegetable binder. Meat doesn't have the right consistancy to be extruded into kibble, so vegetables are added to bind it together.
If there are lots of fruits and vegetables further on in the list (ie. ingredients #8, 9, 10, 11, 13, 14, 16, 17 are vegetables or fruits), it's not much to worry about. They're added to supply vitamins and minerals, often so that the food can meet all of the necessary requirements to be called "complete and balanced". Sometimes they're part of the brand's marketing, to make people feel that the food looks natural and healthy. Maybe not perfect, but they're only there in trace amounts.
"This food has fish in it"
Some people say that fishy food makes ferrets or their waste stink more. This isn't really something to be concerned about unless there are several fish ingredients at the top of the ingredients list.
One or two fish ingredients in the first 6 might actually be a good thing, since fish can have a lot of health benefits (a great source of omega 3's which help the heart, skin, and coat).
"This food is made for cats/kittens"
In general, cat food is made in the same way, and from the same ingredients as ferret foods. Ferrets and kittens have very similar nutritional needs, and it is often easier to find very high quality cat/kitten food than ferret food (never feed your ferret cheap grocery/department store cat food though!). If you want to read a full argument for this, click here
If you're not comfortable feeding cat food to your ferret, then don't do it. It might be worth looking into though, especially if quality ferret foods aren't available in your area.
"This food has very high protein, won't it harm their kidneys?"
That's entirely a myth and misunderstanding. A common TREATMENT for kidney disease in pets is to feed a food with lower protein levels, but high protein levels do NOT CAUSE kidney disease in the first place. That's like saying that you should drink cough syrup every day to keep yourself from catching a cold.
"This food has/doesn't have grains in it"
Grains are bad for ferrets, but so are all veggies. Try to follow this logic:
Yes, all the very best foods are grain-free.
The reason those foods are great is because they have a high meat content, NOT because they're grain-free.
SOME grain free foods have more potatoes or peas than meat. Those are terrible for ferrets.
You can still make a great food with a high meat content that does include grains.
All dry pet food has some vegetable in it. The potatoes, sweet potatoes, and tapioca used in grain-free foods are starchy carbs that are bad for ferrets. Peas seem like they might be a little better, but not by a landslide.
The conculsion: don't pick a grain-free food that doesn't have much meat, and don't avoid a food that has a lot of meat just because it uses grains.
"This food has corn/wheat in it"
Actually, that generalization is kind of helpful. In general, corn and wheat can be very cheap ingredients. Any pet food that is being produced as cheaply as possible will be based on corn or wheat. If you want to weed out most sketchy, cheaply made foods, avoiding corn and wheat is an easy way to do it.
Corn and wheat aren't much technically much worse than other grains or starchy veggies though. If you ever do come accross a really amazing food that happens to have corn in it, don't panic. It's not something I've seen yet though.