Determine lam’s treatment by the last vowel actually pronounced before Allah’s name. Written form alone is not always enough: the actual connected reading or fresh beginning matters.
«قَالَ اللَّهُ» — preceding fathah, heavy lam.
«بِسْمِ اللَّهِ» — preceding kasrah, light lam.
«رَسُولُ اللَّهِ» — preceding dammah, heavy lam.
«اللَّهُمَّ» — at a fresh start, its lam is pronounced heavily.
The contrast shows why continuous recitation and starting afresh must be distinguished. Assess what is actually pronounced, not merely which letters appear on the page.