First off a short history lesson.
The late founder of Ergotron, a man named Harry Sweere was instrumental in convincing LCD monitor manufacturers to adopt an industry standard interface to allow monitors to be attached to the equipment of other manufacturers. He foresaw that as older style CRT monitors were replaced by LCDs, these new, light monitors would become ubiquitous, appearing in all kinds of unexpected places and used for all kinds of at the time, unimagined reasons. And was he right. Hats off to Harry!
Most modern monitors comply with the Video Electronics Standards Association (VESA) interface. Look for a 4 screw hole pattern on the back of the monitor. These holes form a 75mm or 100mm square and are required for mounting the monitor to the WorkFit-S.
If you can't see them, don't despair. They are probably there but covered by or in use by the existing monitor stand. Unfortunately however, some manufacturers of lower cost monitors have started to cut corners and no longer comply with the VESA standard.
If you are not certain about your monitor, you can check compatability here.