I'll admit I haven't done much yet with NS7 or Inventory 7...so I'll have to refer you to an excellent intro article by Doug Jenkins @ Symantec:
Introduction to Custom Inventory in Notification Server 7.0
Depending on how your VBScript skills are, it shouldn't be too tough...something like:
Dim fso, file
Set fso = CreateObject("Scripting.FileSystemObject")
Set file = fso.GetFile("C:\path\to\secret\file.exe")
<all the NS custom inventory stuff from Doug's article, and referencing the custom dataclass you created>
attrib1 = file.DateLastModified
attrib2 = file.Size ' this would be an integer
<all the stuff to write out the custom inventory data>
Once that is done, you'd probably want to enable history on the dataclass.
Another route would be to have a script that pulls these properties from the file and logs them in the registry, then checks/compares them to the previous values. If it does, then set a particular custom inventory with a datetime field in it and a message that gets posted back. Its a little complicated to get into here, but I think there are several ways you could go about doing this.