The conditions for slaughtered meat that is valid to eat are 5:
- The slaughterer must be of the age of tamyiz - i.e. he must be of an age where he is able to intend the slaughter, differentiate between right and wrong and perform simple commands.
- The slaughterer must be either Muslim, Christian, or Jewish.
- The instrument used to slaughter must be sharp and it must slaughter by cutting, not weight or stunning or anything else. It must not be made of teeth or claws of animals.
- The trachea, esophagus, and two jugular veins must be cut for a perfect slaughter. Some scholars say 2 or 3 of the above are sufficient.
- Invoking the name of Allah on the slaughter.
(these conditions are derived from the Qur'an and ahadith and for the most part are agreed upon by scholars of all schools - one difference of opinion is discussed further down.)
So first, ask yourself whether these conditions are satisfied in your case (lunch at workplace, school cafeteria, friend's home, etc.). People unfamiliar with the subject like to tack on extra conditions, but unless they have solid proof they are not conditions. And Allah SWT knows better.
These conditions are merely what makes the slaughter permissible to eat. There are a number of other things that are recommended or highly recommended but not obligatory such as treating the animal well and other things. We are commanded to treat animals well in general and not torture them, and while failing to do so is not part of the sunnah, it will not invalidate the slaughter.
Two common misconceptions are that 1) you can simply say Bismillah over any meat and eat it because the Prophet (saws) did so - the full hadith however states that in that case the meat came from a new Muslim tribe who may not have known of the conditions, and so the Prophet (saws) said Bismillah over it and 2) today's Ahl al-Kitab have deviated from their original religion and it's not valid to eat their food - the answer to this is that they had deviated even at the time of the Prophet (saws) and neither Allah nor His Messenger specified that they must be following their original deen, and so we can't impose that condition either.
The Shafi'i scholars disagree about point 5 above. They say the saying of Allah's name over the slaughter is recommended, not obligatory. For this reason some Shafi'is eat meat where the other four conditions are satisfied. For non-Shafi'is it is usually the fifth condition that invalidates the majority of meat found in stores and restaurants (unless of course it is kosher-certified).
In contemporary Western or non-Muslim societies, there are mainly 2 issues even for those who follow the Shafi'i opinion:
- A good percentage of animals die due to stunning, not cutting. This is particularly the case among cattle - I've heard a figure of up to 30% dying of stunning, before they reach the knife. For poultry, it may be a lower figure.
- The inhabitants of the country are significantly not Christian or Jewish. For example in some Scandinavian countries, the percentage of atheists reaches 70%. Even in England, this number recently crossed the halfway mark. Since this number is not negligible, it calls into question the validity of assuming that the slaughterer most likely of the People of the Scripture. In the US, in some of the Southern states it is well-known that a lot of the slaughterers are Mexican Catholics, and so some Muslims in those areas who follow the Shafi'i opinion eat poultry in those states.
In light of these facts, the permissibility of eating such meat is extremely tenuous.
Another modern-day issue that has not received much attention yet from the scholars is the issue of machine slaughter and whether there needs to be one "Bismillah" per animal, or whether it's permissible to play tapes saying "Bismillah" and other such things.
Source: Summarized from the Precious Provisions course taught by Dr. Yasir Qadhi.