Answer

    thank you for your question :)

How do we indicate the past tense in Chinese?

.... if you want to say about past tense in chinese,you can use the word that show about past tense such as

    yesterday  = 昨天 

    already     = 了

but in this sentense  

* 你怎么了？不舒服吗？

nĭ zěnme le? bù shūfu ma?

หนี เจิ่นเมอ เลอ? ปู้ ซู ฝุ มา?

คุณเป็นอะไรรึเปล่า ไม่สบายหรอ

What happened to you? Were you sick?  = past tense

because  i don't know why you look so sick? then i think you have somethings worng in the past so i ask you What happened to you?