How can I tell the difference between my face flushing and pre-rosacea?

You can't! I'm not even sure pre-rosacea is a diagnosis, but flushing is simply dilated facial skin capillaries in response to emotional triggers, certain foods, alcohol, or other causes not completely understood. Rosacea is characterized by vascular and follicular dilation involving areas of the face and nose; it can be mild to severe, can can involve sebaceous hyperplasia, pustules, and bacterial cofactors.