This is something that your Security Technician (don't know if the term is correct, it's a translation from my country) can help you a lot with and speak with him to set an annual program for conducting this drill.
If you do the post drill assessment alone, then you need to write down the drill steps for a random scenario (fire, flood, earthquake, power outage, terrorism, pandemic, zombie attack!! --> kidding on that etc) as described in your Emergency Preparedness and assess readiness and reaction times of your team to those. Did they get to the speciified exit points, did the assigned crisis officers know what to do and how to guide the personnel through the drill, did the personnel know how to use eg the fire extinguishers, did they use in theory the emergency shut down button of the equipment (if they exist) or just left it working, was an assessement of the safety of your raw materials - end products conducted after the end of the drill in case the scenario included power outage or high heat levels or possible cross-contamination from water (flood scenario) etc., how did you handle potentially unsafe products - products left in the equipment until it was dimmed safe to return to your posts, who and why decided it was safe to return in the facilities, were the appropriate authorities get notified, did the personnel have and knew were to find easy access to the contact details (and even maybe did they call the right one? you don't call the fire department for a pandemic scenario!!).
These are a few examples of what you might need to assess and write down the results of the reactions and preparedness of your staff. How extensive it will be depends on you and the complexity of your processes.