Hi,
one way to do this would be to run "jstat -gc <pid>" where <pid> is the PID of your JVM. jstat is part of the JDK >= 1.5, located in the bin directory. This will give you several columns with the current sizes of all generations of the JVM. The last character "U" always indicates the current usage, "C" the capacity. The current "heap capacity" (which can be adjusted with -Xms and -Xmx) is the sum of S0C, S1C, EC and OC.
Nick.