Given a PDA M such that L(M) is in DCFL construct a DPDA N such that L(N) = L(M)
Take a deterministic TM $M$ and a word $w$. Consider its computation histories for the word $w$. Let $L$ be invalid histories (those which don't start with $w$, don't end with acceptance or are invalid). Either $L = A^{\ast}$ ($M$ doesn't accept $w$) or $L = A^{\ast} - \{h\}$ for some string $h$ ($M$ accepts $w$ with computation history $h$). First of all, $L$ is effective CFL, in the sense that you can build a grammar/PDA recognising it. Moreover, $L$ is a (noneffective) DCFL, but you can't show a DPDA for it effectively. Even more, $L$ is (noneffective) regular.

Small clarification:

You asked if the following problem is decidable:

given PDA $M$ promised that $L(M)$ is a DCFL, and a DPDA $N$ determine if $L(M) = L(N)$. 

The answer is no, and in fact the following stronger fact holds: The following problem is undecidable:

given PDA $M$ promised that $L(M)$ is regular, determine if $L(M)=A^{\ast}$.