。
I know Tom can do it.
Literal
I [to-は] understand-am, Tom [topic-は] can-try-hard [assertive-さ].
Two clauses joined without an explicit conjunction — the first (僕には分かっている) is an emphatic declaration of knowing/understanding, the second (トムはがんばれるさ) is what is known. The split is stylistic, marking emphasis via the pause. 分かっている with には-topic emphasizes 'I, at least, understand.' がんばれる is the potential of 頑張る ('to try hard, to persevere'). さ adds masculine assertive confidence.