When the sails of the windmills in the Netherlands spin, it often means that they are accessible to the public. Here is an overview of rotating mills in the Netherlands.
This website is connected to (currently a few) mills with a sensor. The number of sails passing per minute can be counted in this way. Most windmills have 4 sails, so a number of 60 means that the axis rotates 15 times per minute.
A second plan in development, not yet available: model mills that are linked to real mills. So these model mills will rotate when the real mill is running, at the real speed (up to a certain limit), and stand still when the real mill has stopped. This provides a unique connection with the mill of your choice.