Ultimately the true bellwether of compatibility is the package's own test suite, which we try to get working (and integrated into the build process) for as many packages as possible. For packages which have poor/non-existent test suites, often a downstream package's test suite will expose compatibility problems (we've found bugs in zlib security patches using curl's test suite for instance).

nixpkgs maintainers are frequently the first to notify a project author of incompatibility with new versions of another package.