Then stick it in a matchbox (or ring box!) to stop it from warping.
From The Virginian, by Owen Wister:
*"When the ring was achieved, it was an opal, but set with four
small embracing diamonds. Thus was her month stone joined with
his, that their luck and their love might be inseparably clasped.
He found the size of her finger one day when winter had departed,
and the early grass was green. He made a ring of twisted grass
for her, while she held her hand for him to bind it. He made
another for himself. Then, after each had worn their grass ring
for a while, he begged her to exchange. He did not send his token
away from him, but most carefully measured it. Thus the ring
fitted her well, and the lustrous flame within the opal thrilled
his heart each time he saw it. For now June was near its end; and
that other plain gold ring, which, for safe keeping, he cherished
suspended round his neck day and night, seemed to burn with an
inward glow that was deeper than the opal’s."*
::swoon::
This test is great! I have zero wisdom teeth and I still have an IQ of 480! 
Teach her to use your table saw. When the inevitable happens, BINGO!
Put her finger up your nose
That’s what I did. She didn’t suspect a thing.
You win!
But doesn’t have to be your ring?
Obtain a caliper, or even a decently accurate ruler. Next time she leaves it lying around, measure the inner diameter of the ring (heck, measure it a couple of times from different orientations and take an average). Bring that number to a jeweler, who will tell you what ring size it corresponds to, and then you can buy the ring.
No no no. Put her ring up your nose. Then you can go to the jewelry store and find a ring that feels the same.
What’s a “sunrise”? The best I could find on googling is the name of jewelry companies, but then the above quote wouldn’t make any sense.
In case I’m being wooshed (and it really is a company)… is a “sunrise” anything like “morningwood”? If so, good luck! 
They’re messing around with the OP’s having mistyped “surprise” in his question.
No, no, no.
Put the ring up your hooter then tell her it’s stuck.
Go to jewellers and when he/she ferrets it out just ask 'em to size it when they’ve scraped all the boogers off it.
Easy peasy man
Can you not just borrow the ring-measuring device from the jeweller? It’s like a pointy, graduated spike that you drop the ring down and note the measurement. Would be 20 seconds of surreptitious work at her desk.
The jeweller won’t mind lending you the tool if you’re going to spend some money. I did this for my wife’s wedding ring.
Dear god, man! How big are your feet?! (and you might want to lay off the sauce)
For those of you who don’t remember this thread, let me present, my all time favorite and the reason I joined here.
I stuck my wedding ring up my nose.
Size 10 feet, size 8 fingers and a six pack to get me drunk.
Maybe my tolerance is a little high…
Yeh, maybe the test is a little geared toward typical women.
If your friend frequently leaves her finger lying around, I would be checking the basement for pods before I bothered buying her a ring.
My husband found my ring size (for a surprise engagement ring) this way:
S: Hey Ceejaytee, come put your hand on this piece of paper!
C: Why?
S: My niece is doing a project for school where she has to get the handprints of several people to compare. I want to draw the outline of your left hand.
C: Okay.
S: Now, you do mine.
Couple of months later, a new ring in just the right size. 
I totally fell for it.
Using a chloroform-soaked hanky on her first will keep the elememt of surprise.
I also recomend kidnapping the jeweler’s family to use as insurance to make sure he keeps his trap shut too.
Dynamiting you office as soon as you leave should provide ample distraction.
Good luck!
In my own case it was easy, I just called her twin sister and said “What’s your ring size? Keep your mouth shut!”