[Ludwig Callewaert] wrote in [comp.lang.tcl], regarding [a change-sensitive text widget]:

All changes to the text widget are either done with the insert
or delete text widget command. Overload these (by using [Wcb] for
instance).

By overloading I mean that you provide a new implementation for those
commands. In your case, the overloaded commands could first call
the original insert or delete command and could then set a flag 
(a global variable for instance) indicating that the content of the
text widget has changed. By putting a trace on that flag (with the
trace command), you can make something happen everytime the widget
content changes.

There are actually several ways to do this. One possibility is the
following:

======
 # Create the text widget
 pack [text .t]

 # The flag
 global flag

 set flag 0

 # Put a trace on the flag to execute your proc
 trace add variable flag write changed

 # rename the widget procedure
 rename .t orig_t
 
 # the new widget procedure
 proc .t {args} {
   global flag
   
   set returnVal [eval [concat orig_t $args]]
   set command [lindex $args 0]
   if {[string equal $command "insert"] || [string equal $command "delete"]} {
      set flag 1
   }
   
   return $returnVal
 }

 # Whatever you need to do when something changes
 proc changed {args} {
   puts "something changed"
 }
======

[Wcb], the widget callback package which can be found at http://www.nemethi.de/
is a more general way to do this.

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