I think there are at least two ways that we can convey to our hearers that we are speaking for God, even if we have not said so directly:
In the level of authority that we speak with
"And they were astonished at his doctrine: for he taught them as one that had authority, and not as the scribes."
- Mark 1:22(KJV)
In speaking about the unseen issues of the hearts and motives of men, with the same authority
"I the LORD search the heart, [i]I[i]and
- Jeremiah 17:10(KJV)
