Is my CSPRNG secure? [on hold]

I wrote a seedable* cryptographically secure pseudorandom number generator (CSPRNG) for fun and education in C++.

However, how can I know (or verify) whether my CSPRNG is crpytographically secure?

*Why a seedable CSPRNG? It is an implementation of a pseudo one-time-pad using a password hash as seed and producing a reproducable but (hopefully) cryptographically secure random stream.