How can I get a list of build targets in Ant?

The -p or -projecthelp option does exactly this, so you can just try:

ant -p build.xml

From ant’s command line documentation:

The -projecthelp option prints out a list of the build file’s targets. Targets that include a description attribute are listed as “Main targets”, those without a description are listed as “Other targets”, then the “Default” target is listed (“Other targets” are only displayed if there are no main targets, or if Ant is invoked in -verbose or -debug mode).