How do we know the Lam rule?
Look at the vowel before the Divine Name in connected reading; it is the key to the rule. We do not rely on the shape of the Lam alone.
After fathah or dammah
The Lam of the Divine Name is heavy after a fathah or dammah, as in: قَالَ اللَّهُ، رَسُولُ اللَّهِ.
قَالَ اللَّهُ · رَسُولُ اللَّهِ
After kasrah
It is light after a kasrah, as in: بِسْمِ اللَّهِ، لِلَّهِ.
بِسْمِ اللَّهِ · لِلَّهِ
Starting with the Divine Name
When we start reading with the name of Allah, we begin with a connecting hamzah carrying a fathah, so the Lam is heavy.
اللَّهُCareful in connected reading: The rule depends on the vowel actually pronounced before the Divine Name, so unclear places are left out of automatic grading.