、。
She might be a nurse, but I can't be sure.
Literal
She [topic-は] nurse [might-かもしれない], certainty [topic-は] cannot-hold [but-が].
Two layers of uncertainty stack here. First, ~かもしれない ('might / could be') hedges the main claim. Then a follow-on clause adds 確信はもてない ('I can't have certainty') with clause-final が functioning as a soft 'though / but' — appended after the main claim almost as an afterthought. The contrastive は in 確信は singles out 'certainty' specifically as what's missing, implying that *something else* (a guess, a hunch) is present. This stacking of hedges is a very Japanese way of registering doubt: stating, then qualifying, then walking back. The whole sentence has a thinking-out-loud quality.