From American Rifleman:

In pursuing both personal and professional shooting endeavors over the last several years, I’ve prepared a fair number of new and used firearms for first-time trips to the shooting range. Experience has taught me that one can never assume a gun is ready to shoot until it has been thoroughly checked out. While some gun problems will not present themselves until the gun is fired, many potential issues can be resolved or avoided if they are identified before the trigger is pressed. Here are a few important steps to take before loading a new gun, or a used gun you have never fired: