I have QGIS 3.4.5 32bits. I added to the canvas a shapefile which has no defined coordinate system, but QGIS correctly guesses is WGS84 and warns about it. However when I go to the properties of the layer (right click layer/properties/Source) the Geometry and Coordinate Reference System title indicates that the layer is in WGS84, which is not exactly true since I have not yet actively set the CRS. Is there a way QGIS gives the right information that not CRS is set for the layer? because it is not always the person who created the qgs project, the same person who is using it.
An already loaded layer can not have a non-associated CRS.
You can configure the global (not project) QGIS behaviour when a layer without CRS information is loaded, by choosing it in Global Projection Specification.
There you can configure that QGIS explicitly prompt for the CRS of the layer so that it is actively selected when loading it.