Before the visit
Send your postcode plus the boiler make and model if known. If there is a specific concern, describe it when you message so I know it is not just a routine service request.
During the service
The exact checks and procedures depend on the boiler, its manufacturer instructions, current requirements and the condition found on site. If something needs additional work, I’ll explain that rather than treating it as automatically included.
A routine service is not the same as a repair or breakdown visit. If the boiler has a fault, mention that when requesting the quote.