Hamlet realizes that Rosencrantz and Guildenstern cannot be trusted. What does he let them know in lines 376-377?

What act are you meaning. Hamlet addresses this in Act 2 Sc 2 as well as Act 4 Sc 2. Hamlet basically tells them that the king is using them as spies. I'm unsure where your line numbers are. In act 4 sc 2 Hamlet calls them a sponge,

Ay, sir, that soaks up the king's countenance, his rewards, his authorities. But such officers do the king best service in the end. He keeps them like an apple in the corner of his jaw — first mouthed, to be last swallowed. When he needs what you have gleaned, it is but squeezing you; and, sponge, you shall be dry again.