。
She knows she looks great.
Literal
She [topic-は] herself [subject-が] lovely [adverbial-に] looks [casual-quotative-って] knows.
って is the casual contraction of the quotative と, common in everyday speech and writing — using って instead of と immediately marks the register as conversational. 素敵に見える uses the な-adjective adverbial form (素敵 → 素敵に) plus 見える ('look, appear'), giving 'looks lovely.' 知っている — the resultative ~ている of 知る — is the standard 'know' state in Japanese; bare 知る by itself is rare in this meaning. The sentence has a faint air of 'she knows it and she's working it.'