Question: how to find out the ring size without her knowing?
Answer: Be Subtle. If you are absolutely set on a surprise proposal but you still want to find out her ring size directly from the source, ask your girlfriend in a very subtle, smooth way. ...  Ask Her Friends. ...  Borrow One of her Rings. ...  Leave a Trace. ...  Enlist a Friend. ...  Compare Fingers. ...  Ask Your Partner!