Not at all. I'd say that a great deal (and in some cases the vast majority) of protein in dry dog food comes from
grains.
But there's a big difference between crude protein and usable protein too

Protein that comes from meat sources is far more accessible (digestible) to a dog, and a far superior source of species-appropriate nutrition.
If you take a look at some seriously nasty foods on the market (eg. Pedigree puppy) you'll see that they manage to squeeze 27% crude protein out of a food that's primarily corn. Doesn't mean it's digestible/usable protein though. And that's why you have to feed such a huge amount of foods like that (in some cases more than double the amount of a premium food). Because the accessible nutrition is low.
Really, you just have to make an educated guess based on the combination of information you have available.
- You know they have to list ingredients in order of weight.
- You know that ingredients like "chicken" is inclusive of water content - once that's removed, the ingredient
probably has a rightful place somewhat further down the ingredient list. Note that that doesn't make "chicken" bad - far from it. But if you've got "chicken" and it isn't immediately followed by a meat
meal ingredient, then it's a fair bet that there isn't really much chicken in the food

- You know that ingreients like meat products contain protein. It's not the only ingredient that is a protein source though, so you can't rely on this one alone. But if you were looking at a food that had a couple of meat ingredients in the top five, and then 16 different grains for example, you've got to wonder how much meat is in the food. If you saw the protein content was very low, it would just confirm your suspicion that there was very little actual meat. Or put another way, high protein doesn't prove the existence of meat, but very low protein would prove it's
non-existence. So it's still useful information - but means little on it's own.
Just for the sake of general info. since you mentioned manufacturers disclosing percentages earlier: on this side of the pond, labelling laws do require a certain amount of disclosure (of the top couple of ingredients, anyway). And a reasonably "good" dog food (middle of the range sort of food) like Nutro Natural Choice has about 20% meat. Some of the really seriously nasty foods have as little as 4%.
There are precious few premium foods over here though, so it's hard to say about how much might be in some of the "top of the range" foods. It would surprise me if it's more than about 35-40% in the very best of them though. That's why it's such an issue to see an ingredient list from which you can deduce that there's a decent amount of meat product in the food. Because "decent" still doesn't mean very much at all!
Compare it with a raw natural diet - where the most ardent of vegetable feeders still feed about 80% of the total diet as meaty bones/offal (and most more like 90%), and you start to get a feel for how incredibly low that meat amount in kibble really is.