Run transaction ST02 -> Detailed Analysis -> Click on the 'Storage' button at the bottom of the screen. The Virtual memory allocated line shows the SAP allocated memory.
Be aware that this memory does not include what the database uses or the operating system or other things running on the server. More importantly it does not include heap (private memory) allocated by WP's. It also does not include the executable size of the GW, the ICM and the dispatcher but it does include most of their memory in the shared memory areas.
It also does not allow for additional memory that a Windows system may dynamically allocate to extended memory if the users use it all up.