Say your excuse out loud to an empty room. If it sounds weak there, it sounds weak everywhere.

Hearing something is different from thinking it. In your head an excuse arrives with all its context and emotional backing attached, and spoken aloud it becomes just the words, in order, with the gaps audible. The room has no stake in it, which is exactly what makes the test work.

Do it before the conversation rather than after. Say the sentence to the ceiling and listen as though somebody else had said it to you. If it does not hold, you still have time to go in with the true version instead.