firstly from the tones. Mandarin has four, and Cantonese has six. there's only one of them are the same. others are different. so you can study by this way. secondly, there's no pinyin begin with ng and some others in Mandarin. so you can also learn to know the pronounciation systems of them. then you can tell them out.

actually ,Cantonese is a kind of dialect in China ,and the Mandarian is the formal language ,so in China you can speak Mandarin with anyone ,but only in some place of southern China the people speak Cantonese.

alright, i would rather recommend your learn Mandarin, mostly people in china can speak Mandarin. and Mandarin & Cantonese are quite different.