I don't think there's a need for reconfirmation. The principle of flow cytometry is to use molecular markers, so it must be endothelial cells. Moreover, the antigenic epitope has been occupied during sorting, so it is naturally impossible to perform antibody staining again.

Could you not fix the cells on some glass coverslips and stain for some common endothelial cell markers relevant in your exact cells (maybe ones other than what you stained for facs?).

Also when you mean no success, what does that mean? You are seeing any staining on the microscope?