[Ludwig Callewaert] wrote in [the comp.lang.tcl newsgroup], 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 variable flag w 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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[Arts and crafts of Tcl-Tk programming]