Stache custom helper that can be called with or without block

In the helper function of can.stache.registerHelper, how do I know if the helper is being called with a block or without a block?

2. {{myHelper}}
3. {{#myHelper}}
4.   {{message}}
5. {{/myHelper}}

What would be the test that I would have to call inside the helper function to be able to act differently based on how the helper was called - without block, as in the first call above, or with block, as in the second call?

2. can.stache.registerHelper('myHelper', function(options) {
3.   if (<test here>) {
4.     // helper called without block
5.     return 'I was called without a block';
6.   }
7.   else {
8.     // helper called with block
9.     return options.fn({message: 'I was called with block'});
10.   };
11. });

Thanks.