Rob Allen has a quick new post for you Zend Framework users out there on how to detect if a view helper even exists before you try to use it.
If you need to know whether a view helper exists before you call it, one way is to write a simple view helper to tell you. You can then use it in a view script.
The comments mention another way to do it - calling the getHelper method - but that can cause exceptions to be thrown if the helper doesn't exist. You can find more on view helpers in this section of the Zend Framework manual (or here on custom helpers).