How can you tell if an adult female dog has been spayed?

There's usually a small scar that is visible when the hair on the abdomen is shaved, assuming the spay was done in the most common surgical site (I think there's a thread here somewhere about a dog whose spay was done in a different way)  or you can feel the under the skin sutures. Ginger's was apparently done with metal sutures, as when she's on her back you can see what looks like a wee zipper under the skin. (we adopted her as a 10 yr old, so no clue when her spay was done...long, long ago, I believe)

Well, some have a pretty visible scar. Cherokee's got a scar that's fairly obvious, though I think her incision was bigger than most dogs', and certainly bigger than what they can do today. If the dog doesn't have a scar, or you can't find one? No way to tell at home, I don't think, unless you know what to feel for in their abdomen..but even vets can have a tough time of that.

What you are saying is fine as long as the dog was spayed as an adult.  BUT if the dog was spayed via the laser method at 2 months (or most any time earlier than 4 months typically) you won't have a scar and you won't feel a thing because they simply cauterize the sex organs (on both male and female) with the laser and there is no scar at all.  Even when it's done there is usually only a 1 stitch tiny incision that is barely visible.

In Florida when they do that (and I think all shelters do this, or at least are supposed to) they also chip the dog which indicates it is spayed/neutered).  My Luna is spayed and you can't tell at all that she is externally.

scars can mean c section, and an entire bitch still...so I would not trust anything but an U/S to tell definitatively.

 Some shelter dogs that get early spay are tattooed, some microchipped, some nothing. I would go by what Gina said.  Better safe than sorry.

I agree, you should probably be able to find a scar on their lower belly.  Misty's goes top to bottom, not side ways, so I'm only assuming this is how they're all done, but look for a scar running head to tail.  If she has a little belly, just make sure she's laying flat on her back, it will be much easier to tell that way.

Neither of my dads spayed females have a scar.  They were both done at around 6-9 months.  A lot of it depends on the vet.  I have seen some vets leave big scars and some none at all.  An ultrasound is the best way to go. 

a way they tell with cats is to rub the ab area with rubbing alcohol and it's supposed to help make the scar stand out more. Some animals are also spayed via their side.

I think and ultrasound or opening the dog up for a spay and realizing the dog is already spayed are really the only definitive ways.  Kenya was spayed in April at age three and when I got her at the end of July her scar was still very visible. 

Now, I cannot find it or see it at all.

Emma is 4 years old, and was spayed at 8 weeks. They obviously didn't use a laser on her. She had a whack job of a spay. Her scar is still obvious, and uneven. She is not chipped or tattooed.

Teenie is old as the hills, and I don't know when she was spayed. Her scar is not obvious, but neither is the scar from her recent bladder surgery, when it was discovered that she had no uterus (I'd thought she was coming into season when she was passing stones... BAD mom  I felt so terrible!).

The way I've always been told to do it is wait and see what happens.