。
The kettle must be boiling.
Literal
Kettle [topic-は] is-boiling [certainty-に違いない].
~に違いない ('there's no doubt that') expresses strong certainty based on inference — stronger than だろう, less neutral than はずだ. ~ている on 沸騰する gives the progressive 'is boiling.' The word 薬缶 is a Sino-Japanese compound literally meaning 'medicine can' — historically kettles were used to heat medicinal water, and the name stuck even as the everyday use shifted to tea and cooking.