In my changes to Kermit-20 and SETHOST, I check the configuration byte to make sure that I'm communicating with either a Tops-10 or Tops-20 system.  If it's not one of those, then I want to give a nifty, informative error message, such as:

Kermit-20>connect APOLLO::
?VMS type systems do not support Tops-10/20 NRT communications.
Kermit-20>connect MIM::
?RSX-11M type systems do not support Tops-10/20 NRT communications.
Kermit-20>connect TRON::
?RSTS/E type systems do not support Tops-10/20 NRT communications.

Pretty nifty.  I do this by doing a lookup into a handy table, indexed by the OS type, viz:

hsttyp: eascii <RSTS>           ;^d0
        eascii <RT-11>          ;^d1
        eascii <RSTS/E>         ;^d2
        eascii <RSX-11S>        ;^d3
        eascii <RSX-11M>        ;^d4
        eascii <RSX-11D>        ;^d5
        eascii <IAS>            ;^d6
        eascii <VMS>            ;^d7
        eascii <TOPS-20>        ;^d8 (TOPS20)
        eascii <TOPS-10>        ;^d9 (TOPS10)
        eascii <RTS-8>          ;^d10
        eascii <OS-8>           ;^d11 (!!)
        eascii <RSX-11M+>       ;^d12
        eascii <MCB>            ;^d13 (the DN20!!)
hsttyn=.-hsttyp-1               ; Number of defined operating system types

So if the number I get is outside of this range, I give an unknown error, such as:

Kermit-20>connect ZITI::
?Remote system sent an illegal configuration message

I don't recall whether we used NRT or CTERM to get into our Ultrix machine (it was an 8650, consequently upgraded to an 8700).  I think we used TCP/IP TELNET.