In our country,meat of chicken is found in shop ready made or they are slaughtered immediately before selling.In both cases, should we investigate or ensure in detail during buying whether it's slaughtered Islamically(taking name of Allaah during slaughtering;slaughtered by person who is Muslim,prays,has right aqidah,don't have corrupted view)?If we buy these without investigation,can we cook it?If we cook it ,is it impure & not permitted to eat?

All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.

Since your country is a Muslim country, then you should eat whatever you find slaughtered without investigating about the one who slaughtered it and whether or not it fulfilled the Islamic conditions of slaughter. Also, you should not ask whether the slaughterer has a sound belief or not.

Asking such questions is being obstinate and extravagant in religious matters, which is forbidden.

The action of people should be considered as sound unless one is certain of the opposite. Shaykh Ibn 'Uthaymeen said:

"If we are given meat from the Muslim butchers, then is it a condition for us to know whether or not they had mentioned the name of Allah (whilst slaughtering)? No, although the probability that they did not mention the name of Allah exists. Rather, we buy and eat, and we do not say anything.

This is the constant basic rule: In principle an action done by people is valid unless there is evidence that it is void. When the Prophet said: 'You mention the name of Allah over it and then eat.'; it is as if he is sharply criticizing them; as if he is saying to them: you are not assigned to investigate about the actions of others. The slaughtering is not your action, but it was rather the work of others and you are not responsible for it [you will not be questioned about it], but you are responsible only for what you yourselves do. Therefore, you yourselves say Bismillah (in the name of Allah) and then eat. This meaning is clear for whoever ponders over it.” [End of quote]

Allah knows best.