Saying the name of Allah (the tasmiyah) at the moment of slaughter is commanded in the Qur'an: "And do not eat of that over which the name of Allah has not been mentioned, for indeed it is grave disobedience" (6:121). The majority (Hanafī, Mālikī, Hanbalī) hold it is obligatory but that forgetting it does not invalidate the meat, while the Shāfiʿīs regard it as recommended; almost all agree that deliberately abandoning it out of neglect of the command is a serious matter. When you buy meat and simply don't know whether the name was said, you are not obliged to investigate: the Prophet ﷺ told the Companions who asked about such meat, "Mention the name of Allah yourselves and eat it."
Informational, not a personal fatwa. Consult a qualified scholar for rulings on your situation.