Modern Arabic is written with no "vowel-symbols" (fat7a, dammah, kasrah etc) - you have to learn this on your own.
Or maybe you meant something like "How do I know it's "fa" and not "faa"?" In which case "fa" (like in "funny") is written simply: ف (or فَ if the text has vowel markings) and "faa" (like in "faster") is written فا (a faa with an aleef)
فـ + ا = فا
Still not sure what you were trying to ask exactly though...
I'm guessing that Paolo wanted to ask how he can guess which are the vowels in an Arabic word, given the written form of the word. I seem to remember some threads that addressed this issue here in the forum, but I don't know how to find them.
(1) try to become familiar with the words that you are learning through constant repetition especially the new vocabulary that you are learning everyday
(2) learn all the verbal & nominal pattens & conjugations (3) try and master Arabic syntax which will help you with the short vowels at the endings of words.
You might do well to initially start reading texts that are vowel-marked and then move on to unvowelled texts. You could then re-write the vowelled text but this time without the vowels and then see how you are doing. Generally speaking, to read unvowelled texts is not simple at all at the beginning. It is really only possible if - amongst other things - the above three points are in place. In short, reading an unvowelled text with proper placement of the vowel-markings is the culmination of one's Arabic studies, that is, when everything in Arabic comes together.
Bingo, Abu. I have also just started to learn Arabic as well Paolo, and my approach is this: it's Frustrating!!!! No not really. But I learn to read and write first, this has come through a website primer that thoroughly uses the vowlings properly. Now I am taking a class taught by a teacher who does not use the vowelings, but in learning all the new words, it is clear to me what the vowels would be if they were written down, so I write them down as if they were there. Then, when I memorize the new language, orally/mentally, I figure someday when I know enough words, the grammar, syntax, everything else, I will be able to read the unvowelled text no problemo.
So to sum it up: I learn to read and write first with the vowels, then I learn to speak, then I summate the two, and I will eventually know how to read without the vowels. It's a daunting task, BUT, I would say learning any new language will have its hurdles!
In addition, I would strongly encourage learning or even memorizing:
(1) the perfect form of especially the triliteral verb together with its imperfect, imperative & infinitive forms (and optionally the active & passive participles as well as the passive form of the verb)
(2) the singular form of the noun together with their plural forms
as there are no easy rules for this considering the fact.
Thereafter, learn the conjugations of form (i) up to form (x) of the verb (what they call the 10 forms of the verb) in the perfect, imperfect, imperative, infinitive, active & passive participle as well as passive forms.
Even though this will prove hard at the start, you will find that these patterns enable you to determine from one word seven or eight other words if not more without having seen them before. Many students have found this extremely beneficial and highly encouraging and motivating. This is really the heart & soul of Arabic being a derivation-based language (i.e. from a stem comprising three or four letters you can as a rule derive a number of words which you might not previously have come across).
I hope you find this information useful and wish you all of the best with your journey of discovering the mysteries of the Arabic language.