Sun has a Java System property to determine the bitness of the JVM: 32 or 64:

sun.arch.data.model=32//32bitJVM
sun.arch.data.model=64//64bitJVM

You can use

System.getProperty("sun.arch.data.model")

to determine if its 32/64 from the program.

From thesun.docs:

When writing Java code, how do I distinguish between 32 and 64-bit operation?

There’s no public API that allows you to distinguish between 32 and 64-bit operation. Think of 64-bit as just another platform in the write once, run anywhere tradition. However, if you’d like to write code which is platform specific (shame on you), the system property sun.arch.data.model has the value “32”, “64”, or “unknown”.

The only good reason is if your java code is dependent upon native libraries and your code needs to determine which version (32 or 64bit) to load on startup.