Do the following in the root of your working copy:
svn propget svn:externals -R
As discussed in the comments below, this does not list externals in externals.
Note for TortoiseSVN users: there is an option in the TortoiseSVN installer to also install the SVN command line client. This option is not enabled by default.
与恶龙缠斗过久,自身亦成为恶龙;凝视深渊过久,深渊将回以凝视…