Despite the title of this page, you're not going to understand a lot of Japanese without learning at least a little Japanese. However, there are some audible queues that you can hear when people speak Japanese that can help you understand what someone is saying.
ka: The first is hearing ka at the end of a sentence. In Japanese, this is the equivalent of a "spoken question mark." With very few exceptions, if a sentence ends with ka, someone is asking you a question.
ne: A similar keyword in Japanese is ne (neh) at the end of a sentence. There are a couple common uses for ne in spoken Japanese. If the tone of ne rises at the end of the sentence, the speaker is usually saying "...isn't it?" If the tone of ne falls at the end of a sentence, the speaker is indicating their agreement with you or what you just said.
mashou: There are a couple of types of sentences that end with mashou. The first type of sentence means "let's do something together." For example "Let's eat!" (tabemashou) or "let's go!" (ikimashou). The second type of question is asking if the speaker should do something for you. This is a little less common, but if you're trying to find your way around Japan, you'll sometimes here someone ask they should draw you a map (chizu o kakimashou?)
tekudasai: You will hear this phrase often, however, this one is a little deceptive. This is a combination of a verb ending (te) followed by kudesai. The kudesai is one of the Japanese words for please, and in this case, the speaker is asking you to please do something (or perhaps to not do something). You should be able to tell which from context.
onegaishimasu: This is a critical phrase to learn in Japanese. Literally translated, it means "I beg of you." You'll hear it tossed around casually at restaurants, tourists sites, and the like because it's polite, but when used is normal conversation, it's a serious request. It may be a federal crime to refuse a request followed by onegaishimasu; I'm still researching this. It does come in handy when you're hopelessly lost, though; this is a magical phrase that nearly forces someone to help you. Use it sparingly.