What version of Ppro are you using, what inputs do you have on the monitor? whats the native resolution? what resolution do you want to preview?
Some monitors have a db15 input(VGA), you can use a adapter, some have HDMI you can use a DVI to HDMI cable, if you have Ppro CS4 you can preview to an external computer monitor or TV setup as one or use somthing like a Blackmagic intensity to view previews on a external via hdmi while having two computer monitors for other tasks