Most evacuations will be the same, regardless of its source (fire, water, etc). Your documentation (fire drills, etc.) should specify: the people/organizatins to contact, location of the nearest exit(s) and where people will meet once they are outside.
The only variance would perhaps occur if a bomb threat was announced. Here you would want to inspect/identify the safest passage(s) and use those, rather than all the exits (because you don’t know where the bomb may be placed).
Don’t forget to check places where people may be located that could not be contacted for the evacuation – places such as washrooms, meeting rooms, lunch rooms, furnace/boiler rooms, storage areas, high noise areas, etc. Someone should be appointed to visit these areas to ensure everyone has evacuated the site.
A tool often overlooked is the guest login book. This will show which visitors are in your site, and will also help when you do evacuation head-counts.