There is no detector for a dirty OMAS-Sensor. To clean it is a suggestion if it won't work properly, but this could have some other reasons too. For example, if using transparent media or media with a very smooth backing (e. g. PET), the OMAS Sensor will not work even if it's dirty or not.
So if you have really cleaned the OMAS-Sensor without any results, it could be a problem of the backing of your media.