Let start by saying that in .NET there is strict definition of Integer's size. It is always four bytes. You can run program on 64-bit system and you will still have only 32 bits in Integer. I do like it. No matter which system I run it on, it's range does not change.
Real way to see whether you have 64-bit environment (64-bit framework on 64-bit OS on 64-bit hardware) is to check type whose size does change. That is IntPtr (a.k.a. integer pointer). Although people usually think of integer pointer as something based on Integer, there is slight difference. IntPtr has size of environment's pointer. On 32-bit system it will be same length as Integer, but in 64-bit environment IntPtr grows to 64 bits.
Notice that if your program is executing on 64-bit system, but someone set it's target CPU to x86, this detection will say it is 32-bit (since 32-bit framework is used). Idea behind it is that if you are running in 32-bit, there is no need for you to know whether system is capable of 64-bit. You cannot call that 64-bit code anyhow.
Here is small C# example.