。
I wonder if dinner's ready.
Literal
Dinner-of preparation [topic-は] done-[contracted-てる]-[wonder-かな].
かな ('I wonder') is the casual self-reflective question marker — often used when musing aloud, whether addressing oneself or a companion. できてる is the dropped-い contraction of できている. The whole sentence is thoughtful and informal, like checking to see if the meal is ready without actually walking to the kitchen.