Hole covers are clearly labeled with “HOLE” or by clear visual means (e.g., bright orange engineered cover). 

Okland

Subpart M 11. pg 40

The tops of all hole covers shall be labeled “HOLE” or “COVER”. If the cover is too small to label it, the cover shall be completely coated in orange (e.g. with orange paint).

OSHA

1926.502(i)(4)