I'm pretty sure what your asking is not directly possible.  Java, unlike
scripting languages like Perl, JavaScript and others, does not permit
you to access undefined variables at all.  It's an error at compile time.

If you are having problems like this, you probably want to pass a
(defined) object like a collection, then you can check the contents of
the collection for what pairs of data are defined.  This can get
complicated if you over use it, so be careful.

Also, there's a process called Reflection that does allow you to
discover the type, methods, and other information about objects, without
knowing anything about them to begin with.  This might be useful,
depending on what you are trying to do.

But if "var" isn't defined, then your program won't compile, and you
can't test for it, because it would have to be defined first.