There are many ways to accomplish this:
1. Download the RealTraps tones and plot a graph or import into Excel. You will need an spl meter to accomplish, takes about an hour.
2. Use RoomEQWizzard free software from Home Theater Shack forums. You will need a HTPC and SPL meter and decent soundcard.
3. Use the Velodyne SMS-1 subwoofer EQ system which will create a graph with included mic.
4. Plus many other products available.
In most cases you are taking the readings from the listening position.
3.