Question: how to find out engagement 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!