If you want global information (e.g. your total memory or -Xm' are ok?)
call java with "-verbose:gc -Xloggc:/tmp/gc.log" and you will get a lot of
knowledge on how your system  works. Give "-incgc" a try, but most of the
time it sucks!

If you wana detailed information maybe you can use a memory profiler. Also
Java's system library has 'totalMemory()' and 'freeMemory()' witch can
help too.