The Sims Package Selector allows you to identify .package between sims2 and sims3 packages.
If you mod for both The Sims 2 and The Sims 3, with this utility you will only have to double-click on a package file and it will automatically load into the correct package editor (fully configurable, of course). This utility will associate itself as the default program for opening Sims2/Sims3 .package files.
It will determine if it is a Sims2 or Sims3 package and pass it on to the appropriate package editor. You can open with Simpe TS2 .package files and with S3pe TS3 .package files, in add you can easily identify if a file is made for Sims2 or Sims3 though the icon.