Whenever I look at a job, I examine not only the water heater but the entire system.  This means checking the fuel lines, water lines, the distance between the water heater and the trailer, the distance of the water heater to the ground.  Insulation is very important as well as keeping combustibles away from the heating element.  A certified water heater handyman should both clean and evaluate the whole water heater system once a year.  In most of the United States, no license or certification is required, so homeowners should ask their handyman for an up-to-date certificate from the Water Heater Handyman Safety Institute of America, which requires handymen to pass a written examination based on water heater codes, standards for installation and maintenence.  In addition, you should call references and ask for proof that the handyman has actually worked on a water heater before.  Many municipalities actually have codes that are more stringent than the national fire code, so it is important that your handyman also know the local police, very well.