One of the most frequent notions encountered in regards to Halal food is the idea that one may just say Bismillah on their food and their religious obligations of ensuring the food is Halal are relieved. This is often a result of a misinterpretation of the following Hadith narrated by Aisha (رضي الله عنه):
A group of people once said to the Prophet ﷺ: “Some people have come to us with meat, and we do not know whether the name of Allah has been mentioned over it or not.” He ﷺ replied: “You yourselves mention the name of Allah over it and eat.” 
Although it is often not included, it is imperative to note that in the authentic narration, Aisha (رضي الله عنه) then says:
“They [the ones who offered the meat] had just recently entered Islam.”
The great Islamic scholar Ibn Hajar al-Asqalani, the author of perhaps the most prevalent and prominent commentary on the Book of Imam Bukhari, writes in his explanation of this Hadith:
Ibn al-Tin states:
There is no legal burden [upon Muslims] to verify whether Allah’s name has been mentioned if a slaughtering is done by someone other than them and they are ignorant of whether Allah’s name was mentioned or not. The slaughtering is judged unacceptable only if it surfaces with certainty that the slaughterer did not mention Allah’s name. It is also possible that this hadith means that if the meat comes from someone whose slaughtered animal becomes lawful if he does in fact mention the name of Allah, then mentioning His name before eating makes the meat lawful if you do not know whether His name was mentioned at the time of slaughtering. We can gather from this understanding that all meat found in the Muslim markets is judged to be lawful, as is meat slaughtered by Muslim Bedouin Arabs, because for the most part, they know about the requirement of mentioning Allah’s name. Ibn ‘Abd al-Barr has firmly settled on this conclusion. 
Two points can be extracted from this commentary:
- This default ruling applies only to meat slaughtered by Muslims
- Even if this is extended to meat of “People of the Book”, it is necessary to reflect on the statement, “The slaughtering is judged unacceptable only if it surfaces with certainty that the slaughterer did not mention Allah’s name.” In our current society, nearly 100% of the meat available in the West claimed to be from “People of the Book” does not have the mention of Allah’s name at the slaughtering. Hence, it becomes clear that the ruling of merely saying Bismillah over one’s plate does not apply to our situation.
2Fath al Bari 9:635-636, As cited by Toft, Amir “The Islamic Laws of Animal Slaughter” pg. 66. California: White Thread Press, 2006.