alice, you are not alone.
Most people are confused and many just don't think about or question such things. It's no surprise since it is a rather complicated chemistry process.
The "easy" part of your questions...The amount of protein presented in nutritional data charts represents the amount of protein that the food contains as determined by the method used to do so.
So the best you can do is precisely what you suggested. Go to the charts and add up the protein figures you see there for the amount you are eating.
As for how is it calculated...(I'll attempt to be clear and concise without getting too technical...)
Traditional methods, such as the
Kjeldahl method, of determining the amount of protein in a food is based on the total amount of Nitrogen the food contains which is then multiplied by a determined factor to arrive at its protein content. The problem with such methods is that it assumes that nothing else, such as carbohydrates and fats, contains Nitrogen and further assumes that all nitrogen present in food is from amino acids. Such methods continue to be used to determine the crude amount of protein in food.
A more revised method is to isolate the amino acids by hydrolysis and then measure their amounts by several means (such as liquid chromatography and ion-exchange). Then the sum total of amino acids by weight is said to represent the protein content in the food.
Even with such "better" methods of determining the protein/amino acid content of foods there still remains important questions like the ones you ask...
"Can every body metabolize all the protein found in food into amino acids regardless of the whole food it comes from?"
"What is the actual usable amount of the amino acids in the food?"
"How does food combining effect the usable amount of protein?"
"How do the methods of growing the food effect it's content? Is there a difference between conventional spinach bought in a store that was shipped from Mexico several weeks ago versus the organic spinach I pick from my garden this morning?"Hopefully I didn't confuse you any further