Have you tried using the alarm(timeout) function?

# psuedo code
alarm(someresonabletime)

connect ; # blocks completely if you're prompted for PW

# check error status and if SIG{ALARM} was called
alarm(0) ; # disable pending alarm
if( we got an EINTR or our SIG{ALARM} was called ) {
   broken ssh
}

##
## Connected
##
[download]

The only problem is that you won't be able to differentiate between a broken ssh and a system that's slow in responding.