The biggest consistent tell for LLM writing is when the conversation leaks through into the final prose.

You read along with the text and things seem to be going fine until all of the sudden it starts arguing against a position that no one has actually taken and which doesn't feature elsewhere in the text at all. Then it drops that and goes on for a while before doing the whole thing again about a totally different tangent.

"A tempting option would be to {do this thing that no one would ever actually consider doing}, but it won't work because {reasons}."

You can almost hear the exasperated human on the other side of this conversation telling Claude that it got an idea wrong and then proceeding to not actually proofread the text as a whole before shipping it.