How can one identify words that ought not to be uttered due to their disrespect towards Allah, and to what extent is one held accountable for unintentional speech?

If the speaker intends by the phrase "May Allah prolong your days" a long life and not immortality, then there is no harm. However, it is better to supplicate with words that carry only a correct meaning. But if the speaker intends continuous immortality, then it is impermissible. This is what Sheikh Ibn Uthaymeen meant when he said that it is an transgression in supplication because it contradicts Allah's saying: "Everyone upon the earth will perish" and "And We did not grant to any human being before you immortality." As for whoever utters it without intending any bad meaning, there is no sin upon him, according to Allah's saying: "And there is no blame upon you for that in which you have erred, but [only for] what your hearts intended."