if you start Tomcat from the command line (with startup.sh or startup. bat inside the bin/ directory) it should print some information to the console. There you should see which JRE_HOME Tomcat currently uses.
Additionally it should show up in logs/catalina.out (after a Tomcat start) or you can see it in the command line string (at least on UNIX/Linux).
Another alternative would be a management application like Lambda probe which shows you lots of details about your Tomcat instance and web applications.