I am such a geek I have Title 21 Part 1040.10/11 in a binder.
Warning sign requirements are such:
Class IIIa 2.5e-3 W/cm^2 or less (CAUTION SIGN)
Position 1 LASER RADIATION--DO NOT STARE INTO BEAM OR VIEW DIRECTLY WITH OPTICAL INSTRUMENTS.
Position 2 Laser type information
Position 3 CLASS IIIa LASER PRODUCT
Class IIIa >2.5e-3 W/cm^2 to 5.0e-3 W/cm^2 (DANGER SIGN)
Position 1 LASER RADIATION--AVOID DIRECT EYE EXPOSURE
Position 2 Laser type information
Position 3 CLASS IIIa LASER PRODUCT
Class IIIb >5.0e-3 W/cm^2 to 499e-3 W/cm^2 (DANGER SIGN)
Position 1 LASER RADIATION--AVOID DIRECT EXPOSURE TO BEAM
Position 2 Laser type information
Position 3 CLASS IIIb LASER PRODUCT
Class IV 500e-3 W/cm^2 or higher (DANGER SIGN)
Position 1 LASER RADIATION--AVOID EYE OR SKIN EXPOSURE TO DIRECT OR SCATTERED RADIATION
Position 2 Laser type information
Position 3 CLASS IV LASER PRODUCT
These are the general guidelines set forth by the regulations. There's a ton of specific items to do anything other than general classification. Lots of tables to consult.