Don't pretend to be so confused by context, people. English has many words that sound the same (some even look the same. Some could make sense either way we look at it!), but we understand its meaning by context. Words like "know or no", "hear or here", "peace and piece", "'suit yourself' or 'he wore a suit.'" These are just simple examples, English has plenty more complicated ones.
So "ist" is the word for "is", and "isst" the singular form of "essen" for "eat". In spelling we can see the difference. If we are just hearing it we have to determine is the sentence asking "It is a woman" or "It is eating a woman"? Well, unless we were talking about and animal or something eating a woman, it is not common or likely that were are talking about it eating a woman. So the logical conclusion is "It is a woman".
I read another comment or two where they were talking about being able to write either "Der Mann isst einen Apfel." or "Einen Apfel isst der Mann." Both sentences mean "The man is eating an apple." Is there any reason to think that this sentence means "A woman is eating it?" Or is there a clear reason to not have that conclusion?
I am not entirely sure I understand your question but let me explain why "Der Mann isst einen Apfel." and "Einen Apfel isst der Mann." both mean "The man is eating an apple." Look at the form "apple" is in, in the second sentence. "Einen Apfel" is the in the akkusativ form (direct object, in which the action/verb is being done to the apple), while "man" in the second sentence is still "der Mann" which is nominativ (subject, or the one doing the action). If "apple" were nominativ it would be "ein Apfel", likewise if "man" were akkusativ it would be "den Mann". Oh, I think I may understand your question now. Please correct me if I misunderstand. Perhaps you want to know if it is saying "the woman eats it." but is in a different order as "Einen Apfel isst der Mann." is in a different order. In such a case I would understand your conclusion, and any confusion. Yet, you must think of the situation. In this case we can see that the verb is "ist" (meaning "i"s in English), not "isst" (meaning "to eat" in the er,sie,es and du form of German). If it were a case of it being spoken, you would still be considering context. If you were talking about the woman eating "it" you would most likely know what "it" is and therefore would already know the context of which is speaking. In more cases than not, I believe, if you were to hear the sentence "Es ist eine Frau", it would be "It is a woman." Overall if you were to hear this sentence you would more than likely know what "it" is, that is being spoken of, and as such could determine the context and meaning of the sentence. I hope I understood correctly and could be of proper help to you.
This site should help you a bit. http://german.about.com/od/grammar/a/German-Personal-Pronoun-EsII.htm
Each word has a different "gender" so to speak that effects it's article. Also its case wether the subject (what does the action), also known as nominative, or direction object (what the action is done to), known as accusative. There are two more cases, Dative and Genitive, but right now I'll keep it simple and tell you about nominative and accusative. "der", "die", and "das" are "the" articles. "ein" is "a/an" articles, and it ending varies based on gender and case.
Masculine: -Nominative: "der" und "ein" -Accusative: "den" und "einen"
Feminine: -Nominative: "die" und "eine" -Accusative: "die" und "eine"
Neutral: -Nominative: "das" und "ein" -Accusative: "das" und "ein"
Plural: -Nominative: "die" und "eine" -Accusative: "die" und "eine"
So really, between nominative and accusative masculine is all that changes. So because "die Frau" is feminine it is "eine Frau".
I put "That is a woman" and got it wrong, but in another example of "Es ist ein Mann", I put "It is a man" and got that wrong(!) - the correct answer to that was "That is a man".
So my question is, when does "es ist" mean "it is" and when does it mean "that is"? Is there a way to infer it from this sentence?