Found something:
sun.arch.data.model will return bitmode running Sun or IBM VM.
System.out.println("sun.arch.data.model=" + (System.getProperty("sun.arch.data.model") == null ? "<not available>" : System.getProperty("sun.arch.data.model")));
System.out.println("com.ibm.vm.bitmode=" + (System.getProperty("com.ibm.vm.bitmode") == null ? "<not available>" : System.getProperty("com.ibm.vm.bitmode")));