The list of targets invoked is available in the property
ant.project.invoked-targets
If there is a default target specified, then that will be the invoked target. If one or more targets are specified on the command line, these appear comma-separated in the property. Note that the property only becomes set once execution passes to a target - if you try to read the property outside of any target, it will be unset.
So, if the project has a default target 'zero':
$ ant one two
# ant.project.invoked-targets is set to:
one,two
and
$ ant
# ant.project.invoked-targets is set to (default):
zero