Godot 3.1 will have an is_instance_valid() function that can tell if Object was freed.
It’s used like this:
is_instance_valid( object )
It’s available in Godot 3.1 alpha.
Before 3.1 we can use weakref to hold objects. If object was freed weakref compares equal to null.
You will need a standard reference alongside weakref and change them together.
Not an ideal solution, i know, but at least it does not depend on a comparison to a string that can vary, for example i’m getting “Deleted Object”, not “[Object:null]”.