How can I determine if a form exists in my Drupal installation? I have a form with a simple textfield where the user enters a string. I want to check if this string is a valid form_id in the website. I thought about using the drupal_get_form but even if it would work, it would be kind of an overkill to retrieve the whole form array just to check if it exists. 

Any ideas are more than welcome.
Since the 'ID' you pass to drupal_get_form() is actually the name of a PHP function, the simplest way to see if it exists is using plain old PHP:

if (function_exists('form_id')) {
  // ...
}


If you need to check that the return from the function is a suitable Drupal form array, you'll need to use one of the methods described by MPD.

This will only work for forms for which the name is explicitly defined as the string you're passing in; i.e. forms whose root function is provided by a hook_forms() implementation will not match using this method.

It's worth bearing in mind that with whichever method you use, any form you're trying to identify will have to exist in a file that's included in the current page request. With that in mind it might be wise to keep all the form functions in question in the .module file of an enabled module, which will always be included.