Answer
(Fatwa: 356/D=46/K=1433)
According to Hanafi Fiqh it is necessary to recite بسم الله at the time of slaughtering the animal. It shall not be halal without it. However if someone slaughtered the animal and forgot reciting بسم الله then the animal shall be halal.
لا تحل ذبيحة من تعمد ترك التسمية – فإن تركها ناسياً حل . (رد المحتار : 9/433)
But if it is not known about any animal whether بسم الله was recited over it or not then the act of a Muslim shall be considered on right way. Hence if any Muslim sells meat or he is the owner of any restaurant then it is obvious that he must be using halal meat and it is lawful to eat it. It is not wajib to investigate or enquire about it.
إن قوما قالو للنبي صلى الله عليه وسلم إن قوماً يأتوننا بلحم لا ندري أذكر اسم الله عليه أم لا إلخ . (الحديث)
But in view of the carelessness of present time if someone wants to take care and out of taqwa avoids, then it is far better. However if it is known that he has missed بسم الله knowingly then the animal shall not be halal.
It is not necessary to recite بسم الله at the time of slicing fish.
Allah (Subhana Wa Ta’ala) knows Best
Darul Ifta,
Darul Uloom Deoband