What are the variables does one search for when trying to render a custom form?
To answer your explicit question (and to reiterate what Clive said), there's no one thing that you could grep to conclude the arguments to change for the getStorage() &amp; create() methods. 

The above solution assumes:


You're looking for an entity form.
You're aware that EntityFormBuilderInterface::getForm() needs an entity object.
You can build an entity object with Drupal::entityTypeManager(). 


It's a specific solution to building entity forms. If you're looking for a method to find any arbitrary constructed form, this solution isn't going to help here.

The most generic thing one could search for to find such a form (an invoke it for rendering), is probably a label that is unique to the custom form, but it really depends on how the form is constructed (e.g. EntityFormBuilderInterface::getForm(), FormBuilderInterface::getForm(), or some arbitrary form array builder function). 

If you're familiar with the pattern of construction, there might be some specific terms to search for (e.g. searching for classes implementing FormBuilderInterface or EntityFormBuilderInterface), but there's not going to be one search pattern that works for all forms.