Grounded light switch box with no ground screw/wire
First, is the box actually metal?  If not, forget about it.  

Most boxes are tapped for a #10-32 screw for a ground screw because as a matter of convention and industry practice, that is what everyone uses.    I believe the minimum screw size is #6, however -32 is the coarsest allowed thread pitch.   That is because UL standardizes box thicknesses, and this assures enough thread engagement to be effective.   -40 is also allowed.  A sheet metal screw is out of the question.   A self-tapper is ok if it is -32 or finer.  

If you do not see a ground screw hole tapped, either the box does not have one (some don't), you're not seeing it because you expect it to be on a dimple and it's flush, or the installer happened to use that hole for a mounting screw/nail. 

You are welcome to either use  a ground clip, or drill and tap a 10-32 hole yourself.   Use the correct drill listed with the 10-32 tap.