。。
It's true. You'd better believe it.
Literal
True [assertion-さ]. Believed [option-方] [subject-が] good [emphatic-ぜ].
Two very masculine sentence-final particles in succession. さ after a noun is a casual assertion/explanation marker, roughly 'you see / I tell you' — confident and often masculine-leaning. ぜ is a strong, swaggering masculine emphasis particle, more assertive than よ and rarely used by women. 信じた方がいい ('you'd better believe,' literally 'the option of having believed is good') uses the ~たほうがいい pattern: past-tense verb + ほうがいい = advice/recommendation. A classic pattern that surprises learners because the 'past tense' is not actually past here.