I know that at the beginning of a noun, when the hamzat-al-wasl in the definite article الْ is pronounced, it is always pronounced as "al-" (i.e. with a fatha). However, using Al-Fatihah 6 as an example, the hamzat-al-wasl in اهْدِنَا is pronounced "ihdinaa" (i.e. with a kasra).
Visually, there is nothing to distinguish the hamzat-al-wasl in either case; they are both written exactly the same. How should one know which vowel to pronounce when the hamzat-al-wasl comes at the beginning of an ayah (i.e. is not absorbed into the preceding vowel), or is it just a matter of learning and memorizing how each word would be pronounced when it comes up?