Normally things on the crontab are tested so, all the output of it is redirected to /dev/null. You can debug it by redirecting it to a file; for example
* * * * * (useradd adrian) >>/tmp/debug-useradd 2>&1
All the messages will be appended to this file in the /tmp directory. As you say you already tried "/usr/sbin" in Woody, it is probable it is in another directory. You can determine the real path with "which useradd" or, in the worst of the hypothesis, "find / -mount -name useradd".
By the way, you got it right.