According to the Japanese dictionary "Dai-Ji-Rin", Original usage of "wa(は)" is that pick up and show one thing from several things. "ga(が)" show nominative case, and show the target one for hope, the ability, and likes and dislikes, etc.(This is from my translation) I tried to give you some example. "Watashi wa Ringo ga Tabetai" means "I want to eat some apple". It has been focused on "Watashi" from some people by "wa" of the sentence. It show the target of "Tabetai" is "Ringo" with "ga" of the sentence. then... "Watashi wa Tabetai" is the sentence which deleted "Ringo ga". We feel some GAP at the point which is between "Watashi wa" and "Tabetai". When you will tell us "Watashi wa Tabetai" without "Ringo ga", we will ask you what do you want to eat. "Ringo ga Tabetai" is the sentence which deleted "Watashi wa". When you will tell us "Ringo ga Tabetai" without "Watashi ga", we may ask you who want to eat some apple. If you will tell us "Watashi wa Ringo wa Tabetai", we may ask you what do you want to eat for another.

I think you can just understand from the context of the sentence. Like in: "watashi-wa nihongo-ga dekiru" | "I can speak Japanese." It is easier to tell when you understand the difference between "wa" and "ga". I like the explanation on this site: http://www.guidetojapanese.org/particles.html