。
I wonder if she's married.
Literal
She [topic-は] is-married [explanatory-の] [I-wonder-だろうか].
結婚している looks like 'is currently in the act of marrying' but isn't — 結婚する is a punctual verb (the wedding is an event), so 結婚している takes the resulting-state reading: 'is in the state of having married' = 'is married.' Same pattern as 知っている ('know'), 死んでいる ('be dead'), 結婚している. Tacked on at the end, ~のだろうか combines the explanatory の (here softening the inquiry into a thoughtful musing) with だろうか, the masculine/neutral form of 'I wonder if.' Together they deliver a contemplative 'so… is she married?' rather than a direct question.