Mandarin have no grammatical tense markers, it use markers of aspect ( eg. 了, 過) or time adverbial ( eg.曾經, 幾天前) to express. Wiki has a introduction about Mandarin Aspect: http://en.wikipedia.org/wiki/Chinese_grammar#Aspects

The context, a few verbs, some particles and the expressions/adverbs of time are all that you need to tell when the action takes place. Please find a few examples below. Adverbs of time: 我昨天回家时，...... When I returned home yesterday,... 我今天回家时，...... When I return(ed) home today,... 我明天回家时，...... When I return home tomorrow,... Verbs and particles: V+guo 过 This suffix generally indicates that an action has taken place, but does not continue up to the present. It's used to talk about things that happened at least once. Have you been to Beijing? 你去过北京吗? V+了 Something was accomplished / completed in the past. 我在两周前到了。I arrived two weeks ago. (and I'm still here) V+了 can also be used to express the near future. You're knocking on the door and a voice says from inside the house: "我来了!" I'll be there! (= I'm coming!) 将+V to be about to V 明天我将去看他。I'll visit him tomorrow. 会 = likely, probable 明天会更好。Tomorrow will be better / might be better. (正)在 = present continuous (the action is being performed right now) 我(正)在吃飯呢｡ I'm eating.

by situation, judging from time indicating words