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You normally cannot determine that without exposure. You would have to check a dictionary. The most common one in English for students of Arabic is probably Hans Wehr, available to browse at the top of this link. It's also available as an Android app.
In the Hans Wehr dictionary, the letter (a, i, or u) which appears after the transliteration of the third-person singular masculine conjugation indicates the vowel of the "عين الفعل" (second root-letter) in the imperfect (muḍāriʿ) form, assuming no or before any verb irregularities.
heres the link... https://play.google.com/store/apps/details?id=com.etjaal.arabicalmanac
ALSO... this is one of my favorite arabic-english dictionaries... a lot of content in here...
get familiar with hans wehr
if you're on android this is a good app for it https://play.google.com/store/apps/details?id=com.etjaal.arabicalmanac&hl=en&gl=US
if you are on android this is a good app for hans wehr https://play.google.com/store/apps/details?id=com.etjaal.arabicalmanac&hl=en
I use it every day.