The function python_is_optimized() in Lib/test/support.py:

def python_is_optimized():
    """Find if Python was built with optimizations."""
    cflags = sysconfig.get_config_var('PY_CFLAGS') or ''
    final_opt = ""
    for opt in cflags.split():
        if opt.startswith('-O'):
            final_opt = opt
    return final_opt not in ('', '-O0', '-Og')

However, it seems that the *default* (when no special CFLAGS are configured) is -O3. My Python build is done with this default -O3 but python_is_optimized() returns False. This is the reason why I didn't catch the buildbot failure at
https://github.com/python/cpython/pull/13185#issuecomment-497062965
(this test is only run when python_is_optimized() is True)