How to kn if plaster is dry I paint over

If it is a uniform light pink colour all over then yes it is dry. Pay particular attention to the corners, this is where the plaster drying process is at its slowest due to air circulation. If you use the correct paint for new plaster ( as i am sure you are planning to) then that paint will allow some moisture remaining to escape as it does not dry with a paint film. But it is always best to wait untill the plaster is 100% dry

It is right when light and pink it is showing the drying process. But not always dry, normally a good week and ventilation will be enough, unless solid plastering will take twice the time. The ceiling line and internals will be last to cure, a good rule of thumb is to rub a fine piece of sand paper in the corner sand if it is damp on the back then it is not ready to paint. As for miss coat as long as you water down to milk consitancy or use micro poures paint it will be fine, leave for a day then paint as normal.

Any cheap white paint mixed with water will do.