where java
works for me to list all java exe but java -verbose
tells you which rt.jar
is used and thus which jre (full path):
[Opened C:\Program Files\Java\jre6\lib\rt.jar]
...
Edit: win7 and java:
java version "1.6.0_20"
Java(TM) SE Runtime Environment (build 1.6.0_20-b02)
Java HotSpot(TM) 64-Bit Server VM (build 16.3-b01, mixed mode)