Taken from the Canine Reproduction, Fertility and Neonatal Challenges Page
So very much of what we do is deadly serious and we stress over every little thing when we are doing that breeding which we have thought about and planned for such a long time. Everyone always wants to know "IS SHE PREGNANT?" and "WHAT DID YOU SEE ON THE ULTRASOUND??" So the "ring test" is the grins and giggles answer that you can do yourself, no extra cost! And if you want you can not only do it every day up to delivery, but you can do it to see what she has left during the whelping pause. All you need is a ring, you could even use a washer or nut, some string or thread and lot of belief, and a bit of patience! Put the ring on about 12-18" of string or thread, make sure it is secure. Gently lay your dog on her back, remember bribing her with treats is part of the routine!! I found that it is easiest to do when my girl is laying between my outstretched legs on the floor. You then work your way slowly down each side of your girls belly. You want to let the movement of the string/ring "show you" what is there. If the ring goes in a circle it is a GIRL, if it goes in a straight line it is a BOY! (This is the opposite of what it is when you do it on pregnant people!) Here is the YouTube that shows you how it is done! Tiffany who made this YouTube does it a few days after breeding (3-10) and then again at 3-4wks post breeding. I just did it a week away and am biting my nails to see if I am any good at this! Remember if the ring goes in ROUND CIRCLES=GIRL, and STRAIGHT LINES=BOY. I believe the text is backwards on the YouTube (so is her voice) so until it gets corrected this is the way to do it.