It is necessary to balance between taking precautions to only eat halal (lawful) food and giving others the benefit of the doubt. So if you visit a store that has their meat labeled halal or you visit a friend whom you know only eats halal meat, then you should not ask. You can safely assume that they are being honest and not deceiving you. However, if you visit a store that does not label their meat, or you visit a friend whom you have a reasonable suspicion that they do not only eat halal meat, then you must ask. You should be polite, but not shy away.
Shaykh Mustafa Umar