There is an easy way to check whether iray uses CPUs and/or GPUs: the mental ray log file.

In Customize -> Preferences check the option 'Show/Log Information Messages' and 'Write Messages to File' and select the file you'd like to file to be written to by clicking on the File button:

Note that the actual file name will be max_Production.log even though i selected max.log as the filename.

CPU only

Once the render starts you can look at the log file, scroll down a little bit (or just search for IRAY) and you'll see something like:

IRAY 0.3 info : using CPU backend with 4 threads

if you don't have a CUDA capable graphics board in the machine. In this case it's rendering on four threads (the machine has two cpu's and hyperthreading is enabled).

With a CUDA capable graphics card

Or something like this if you do have a CUDA capable graphics board:

IRAY 0.3  info : found 1 CUDA devices


IRAY 0.3  info : defaulting to use all CUDA devices

IRAY 0.3  info : using CPU backend with 8 threads

IRAY 0.11 info : CUDA device 0: "Quadro FX 1800M" (compute capability 1.2)


Both the GPU and CPUs are used. You may need to update your driver, check the 3ds Max hardware page.

Asking for the impossible...

If you use the Iray Manager Maxscript to force the use of the GPUs on a machine without a CUDA capable graphics board you get:

IRAY 0.3 warn : no CUDA capable devices found

For more information see the iray FAQ on Shane's blog. Check whether your graphics card is listed on http://www.nvidia.com/object/cuda_gpus.html