I don't understand this question. Are you really asking whether it's possible to find out how much glutamic acid is in a gram of a given foodstuff? The answer is "yes", of course, just like you can easily find out how much sodium or iron is food.
How? I don't see a 'grams of free glutamate' label on food. I can't even find sources that tell me the average amount of free glutamates in foods where MSG is typically added.
This is exactly the problem the parent comment describes. Manufacturers aren't required to even mention MSG, or any glutamate for that matter, so we simply don't know.
