public class JavaArch{
        public static void main(String[] args){
                String arch = System.getProperty("sun.arch.data.model");
                System.out.println("arch:"+arch);
        }
}

Windows:

F:\>"C:\Program Files\Java\jdk1.8.0_261\bin\javac.exe" JavaArch.java

F:\>java JavaArch
arch:64

AIX:

# vi JavaArch.java
# /usr/java14/bin/javac JavaArch.java
# /usr/java14/bin/java JavaArch
arch:32

# /usr/java5_64/bin/javac JavaArch.java
# /usr/java5_64/bin/java JavaArch
arch:64

# /usr/java14/bin/javac JavaArch.java
# /usr/java5_64/bin/java JavaArch
arch:64

# /usr/java5_64/bin/javac JavaArch.java