It's probably best if you start by stating the actual problem that you're trying to solve, and what your level of experience with Smalltalk is. You might get more helpful answers than if people need to guess those things.
In general, Smalltalk does not have a concept of "ownership", only references. So if an object A has a reference to an object B, it does not own it, there may be other objects who also have a reference. If you're talking about variables, those are the places where references to objects are held, but they are not objects in themselves (with the exception of shared variables which are basically a level of indirection).
That said, there are methods by which you can find which objects point to a given object. These can be used do debug situations in which an object still exists although you thought it should have been garbage collected, but they are not to be used in application code.
If your application requires an object to know who references it, add an explicit instance variable. For example, if you're building a tree data structure consisting of nodes, a common pattern in Smalltalk is to have instance variables "children" and "parent" which are maintained to stay in sync, so that each node in a tree knows its own children and its parent.
Cheers,
Hans-Martin