Weirdly enough, I can’t find a pure-I7 way to do this. But it’s an oversight that’s easy enough to remedy with a tiny bit of I6.
Definition: a snippet is empty if I6 routine "SnippetEmpty" says so (it contains no words).
[ SnippetEmpty s ;
return (s == 0);
Now you can just check “if the topic understood is empty”.
(Well, you can do it with pure I7, by checking “if the number understood is 0”, since an empty snippet corresponds to the number 0. But breaking type-safety by looking at the wrong “the ___ understood” is a great way to make your code opaque and unreadable.)