There are two ways to do this. 1) When we speak, we tend slur letters together. For example the word "hums". We never say hum-ssss. We say the m and the s together humssss. We do this even when we are transitioning between words: "I can speak." When speak naturally, we never segment the words "I-can-speak." It is more like "I-canspeak." There are only certain combinations of letters that this works with: S/N or N/S- It is new / It isnew. I can speak / I canspeak. S/M or MS - Where's mom? / Where'smom. Ham sandwich. Hamsandwich. etc. However, this doesn't work very well with the letter "T" and you will almost always here a break between a word that ends with T and the word that follows it. So, where an affirmative you will hear no break between words: I canspeak. In the negative, you will hear a definite break. Even if the T is not pronounced clearly, the N will sound truncated: I can't speak. 2) The easiest way to distinguish is how much emphasis they put on the word CAN'T. An affirmative will almost always flow without emphasis. Because it is usually used for general information. I can speak German. However, when speaking in the negative, you are usually clarifying rather than giving general information and so the "can't" is almost always emphasized. "I'm sorry. I CAN"T speak German. I hope this helps.

The context helps. Also when in doubt, you can always clarify (Excuse me, did you mean can speak or cannot speak ?). You need to listen to more of English to pick out words like these. Hear music, see serials and shows in english and watch english movies :)