DMDX is another psychology experiment package but this one has the advantage of being free. Unfortunately, unlike the others it has to undergo some configuration after installation.
This video demonstrates how to install and configure DMDX. It's not mentioned in the video but on most systems selecting "Select Video Mode" will automatically select the current resolution of the monitor. Thus, if you're using an LCD screen (and it's configured correctly) you can determine what your monitors native resolution is from here.
Apologies for the audio/video synchronisation problems that occur just after DMDX is installed. Somehow it gets back in sync not long afterwards so I decided to leave it as is.