Getting your Ring Size Right
The best way? Being measured by a professional jeweller. It should be free and can be done in a lunch break.
If you can't get to a jeweller you can approximate your size by cross referencing a ring that already fits well against a sizing chart, making sure you print the chart out at lifesize scale (100% size).
There's a good chart available on the Michael Hill website: Ring sizing chart
Measuring a Ring Size in Secret
Just so we're clear, this technique is not terribly accurate - it can give you a reasonable ball park guess though. Make sure you're measuring a ring that fits on the right finger :)
You'll need to be able to get to a jeweller's to use this method: put your partner's ring on your finger. Pick a finger where the ring fits as close to a normal position as possible. Make two marks, one on either side of the ring. Leave them there and go to a jeweller and tell them you want to find out what size ring goes on your finger where these marks are.
Make sure it goes past a knuckle as this is eventually going to happen with the real ring. This method will give you a pretty good guesstimate, good luck! x