I don't think that this is a standard, but in instances like this I have always used the following text:
FOR CONCEPT ONLY
DO NOT MANUFACTURE
I include this as a diagonal watermark across the page.
Revisions of Drawings, ASME Y14.35M gives you several tools:
A document identification number drawing
XXXXX - AAYYYYYYYYY – INT SUFFIX IDENTIFIER (DASH) Use initial o pre-release.
On the general notes:
INITIAL DRAWING NOT FOR MANUFACTURE
In the revision history table:
INITIAL DRAWING for reference only
Note: must be reference dimensions (xxx) only.