If you are starting Tomcat with startup.bat, it will execute catalina.bat - look at the starting comments in catalina.bat to see all of the environment variables affecting how the JVM is started. You will see (among others)
So - Tomcat will use the JRE version that you set in catalina.bat
Running Tomcat as a service has a different way to control the JRE - sorry, I have no idea how.
If you're running it as a service and have installed it from the '.exe' version, there is a Tomcat configuration app that can be reached from the Window's Start Menu. Under the "Java" tab, there is a field for setting the JRE location.