Amino acids are the building blocks of proteins in all living organisms. Humans, like all animals, must obtain amino acids by eating. Most amino acids can be transformed from other amino acids. However, out of the 20 amino acids that are used, there are 10 amino acids that cannot be obtained by humans via transformation and hence must be obtained via eating. They are arginine, histadine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan and valine.
Most vegetable (non-meat, non-egg, non-dairy) proteins do not contain a well-balanced amino-acid profile of the 10 essential amino acids. One can check the amino profile imbalance by simply looking at a food item's protein score and the different essential amino acids contained in it. The only vegetable sources that contain a well-balanced amino acid profile are quinoa and amaranth seeds. Most vegetable proteins may have a deficiency of lysine or methionine or both.
There is a way for you, as a vegan, to obtain sufficient amounts of vegetable based proteins with a well-balanced amino acid profile even if you don't always consume enough quinoa or amaranth seeds. That is, you can complement a vegetable that is deficient in certain essential amino acids by consuming some other vegetables that have an abundance of those essential amino acids.
For example, fava beans have a protein score of 84 which is not too far away from quinoa or even eggs. However, it does not contain enough methionine (a sulfur amino acid). Therefore, you may want to complement fava beans by eating something like walnuts in the same week that you eat fava beans. Whereas walnuts has an abundance of methionine and a deficiency in lysine, fava beans has an abundance of lysine and a deficiency of methionine. Therefore, fava beans and walnuts complement each other.
Another strategy for a vegan to obtain proteins with a well-balanced amino acid profile is simply to diversify the different vegetables that you consume. Do a lot of mix and match everyday and every week.
Notice that the so-called "complete or well-balanced amino-acid profile" is something developed by some nutritionists who compare the amino-acid profile of a food item with that of an egg. There is no reason to believe that eating egg protein is "perfect" for humans. Plus, there is no one single standard for the composition of human body proteins. Humans are all different with different levels of tolerance. So, don't be too concerned if you have a slight deficiency in a certain amino acid in your diet!
You can check the essential amino acid profile of a particular food item by checking websites such as: