In a poem how do you know when syllables are stressed and when they are unstressed if there are no markings?

Stressed syllables are the same regardless of whether a Word appears in a poem or in prose. You don't pronounce a Word differently just because it happens to be in a poem (unless the poet is taking liberties in order to force a rhyme, when it will be obvious from the context). If you make a special effort to stress certain syllables unnaturally you will end up sounding very mechanical and boring. Poetic rhythm is light and flexible and comes from the natural stresses of the Words: the poet (if she or he is any good) will have seen to that.

If you want to read a poem aloud in a language that you don't know, you will have to ask someone who does know the language to help you. There are no general rules that you can apply.