Ensure the water level in the boiler sight glass is appropriate to prevent dry firing or carryover.
If you are looking for in-depth technical guidance, Nehemiah Hawkins' classic text, Maxims and Instructions for the Boiler Room , is highly recommended, as noted in the search results. Boiler Room
Inspect gas lines for leaks and ensure safety shut-off valves are functional. Ensure the water level in the boiler sight