it's just like DKM and Steve have mentioned, read the nutrition side of the flour bag. Look for the protein content.
In "All Purpose Flour", the protein content varies greatly; from 8 % to 11% or more. The same is for Bread Flour with up to 12 or 13 %.
Pay attention to the ash content as well, high ash flours have a high protein level, but a percentage point or 2 protein comes from the outer layer of the wheat grain. The protein there is of a lesser quality and does not form as much gluten as the protein from the inner of the grain.
I believe King Arthur Flours have the ash content on their labels, not sure if the other brands mention that. Here in Germany, Austria and France, flour is classified according to their extrusion level (equiv. ash content). Ash levels of 0.40 up to 0.65 make good all purpose or bread flours.
A good example of a Flour with high quality Protein is Sir Lancelot from King Arthur. Has 14.0 % Protein and 0.52 Ash.