How did Latins distinguish short or long vowels when they read a text? Does exist any rule such as open / closed syllable?
Vowel quantity is an aspect of spoken Latin that is typically not represented in writing. This is the case today, and was in the antiquity. I find this to be no different to how in many languages (like Russian, Italian, and English) stress is a significant feature of the spoken language but is not indicated in writing. Writing is only a hint to pronunciation, and proficiency is needed to turn text into speech.
The Romans did have ways to mark long vowels, but they rarely used them. In modern contexts macrons are typically considered supporting material for learners. A fluent speaker of Latin, be it in ancient Rome or in today's world, is simply expected to be familiar with the quantities of most vowels in the words and endings.
Over time a Latin learner develops a good sense of what is a likely quantity, but nothing is foolproof, not even to a native. So, in answer to your question, the Romans didn't distinguish long and short vowels in writing because they didn't feel a sufficient need to do so.