Who is asking you to do this?
What is your role in the company?
Why do I ask? It just seems as though if a manager is asking you to do this it would appear that the safety culture is not that important to your company. Wouldn't management know what your safety culture is?
Or, is management attempting to find out if the hourly folk are truly buying into your program? First thing, imho, would be to look at your injury and illness numbers. Where to they fit with the rest of your industry? Then, look at one of the safety program/certifications that are out there. There is the Z10, BS18000, various state and federal OSHA VPP(STAR) programs. Get yourself a copy of one of them and then start looking to see how close you come to meeting it.
Do you have a safety team? (my definition of this is mostly regular workers, feeding information into your higher level management safety committee) Are they active?
It doesn't take a lot to "assess the safety culture", what I'd be asking is what their definition of safety culture assessment is.