To see that it resolves back to the IP address from which the connection is made. It's a standard check that many servers do, and ssh does too (and gives off a warning if they don't match). I'm not sure if it does it in a way that's vulnerable or not, though - but it surely does so by default.

From man sshd_config:

     UseDNS  Specifies whether sshd(8) should look up the
     remote host name and check that the resolved host name
     for the remote IP address maps back to the very same 
     IP address.  The default is “yes”.