There are three main ways you can find out your partner's finger size through stealth, but there are all sorts of variations on each...
Obviously this only works if you're willing to let someone else know your plans but it can be very effective as it seems to have nothing to do with you. If a friend has recently got engaged perhaps she could try the ring on your partner & see if it fits?! Or a day of girly shopping could involve trying on jewellery & getting fingers measured at the same time... Perhaps mum / granny has a ring they know is a good fit?
While she's out, get to a jewellers to get it measured. If you can't do this, try it on yourself and see how it fits you, and get your finger measured instead! Pressing the ring into blutack & carefully taking the impression to a jewellers (without distorting it!) will also work as they can measure from that. Putting the ring on a tapered candle and marking how far it goes is also very effective. There are also size guides that you can download where you hold the ring over a range of printed circles to get an idea of size from that – just make sure the printer doesn’t scale the image strangely, and get rid of the evidence afterwards!
These techniques are best if you know which finger she wears the ring on as different fingers are likely to be different sizes, even varying from left to right hand. Wider rings will also feel smaller than narrow ones so if you can, try to go from a similar width ring to the one you are intending to buy.
We have had lots of customers who swear that they have managed to do this! You need someone who sleeps heavily for this to work! Wrapping a strip of paper / length of string around the finger and marking the circumference will give us a good idea of size, but you need to be quite accurate to do this so it won't be easy in the dark...
Some of my other favourite techniques I have heard from customers include taking an impression of the partner's hand 'for a design project' and then making a wax model to measure; and photocopying the partner's hand & taking the image to be measured. These both required a certain amount of acting ability so aren't for the faint hearted, but they do make a fab story to add to the romance of the proposal!
Let us know if you come up with anything else- we're always delighted to hear what you have achieved and to share your ideas with other people in the same situation.