If you are running Eclipse with PDE (Plugin Development Environment), hit ALT + Shift + F2 to use the Plugin Menu Spy. The cursor will change its' shape, now press the desired perspective button and you will get the perspective ID.
Another option is to run a plug-in search (from Search → Plugin). Look for the string "
org.eclipse.ui.perspectives
" which is the name of the extension point that defines a perspective.
If you don't have PDE, you can search for the same string in the plugin.xml files which are found inside the plugins JARs. If you have a tool that can search within JARs, that would be helpful. Otherwise, you will need to guess which JAR to open for the search (or open all the JARs).