Going to https://status.nixos.org/ is the best way to get started. You
can see that nixos-unstable currently is failing to build. However after
clicking the links you will find that it is an infrastructure
failure. (It
would be very helpful to fix, but I don’t know if that is what you are
looking for).
There are also non-blocking hydra failures, the best bet to find out
about that is probably reading the last call for fixes:
ZERO Hydra Failures 20.09 · Issue #97479 · NixOS/nixpkgs · GitHub.
Another option to help out is finding broken packages and seeing if they
are worth fixing. For example you can find a lot just using GitHub Search
or Sourcegraph.
That’s all I have on hand, hopefully it helps you get started.