What is the ruling on eating in restaurants or from foods that are suspected of not being halal, especially when there is difficulty in verifying their ingredients?

The fundamental principle concerning things is that they are permissible and pure, so nothing is deemed forbidden or impure except with a legal proof. Food items that are doubtful are permissible unless the doubt reaches certainty or strong presumption. It is permissible to abstain from them out of piety. However, if it is certain or strongly presumed that they contain an unprocessed forbidden substance, then they are impure and forbidden. Products of unknown status or source are generally permissible, due to widespread affliction and the fact that most of them have transformed from their original state through processing. If the fundamental principle conflicts with what is prevalent in a country where unlawful slaughter is common, the prevalent situation takes precedence to avoid suspicion.