To clarify, the last rib is the one before the waist. On a pet dog, pinching an inch on the first rib right after the shoulders, is okay. LOL
We may be having a miscommunication on the "easily" concept. I can't think of a, objective way to relate this. On Maggie, my thick-coated dog, I dig my fingers into her fur (not push through her fur) in the place where I know her last rib and waist should be, near her spine. I should feel the rib (the hardness of it) under my fingers at the pressure it required to get through the fur, and should have to go hunting for it.
Then I slide my fingers back, noting the amount of waist (or lack thereof - her usual problem!), until I reach the pelvis. There should be good muscle in the femoral area, and higher up, again, the pelvic point should not require search-and-rescue at the same amount of pressure it took to get through her fur.
Finally, I touch her back between her shoulders. On Maggie, this is the easiest place to find fat first. Other dogs deposit fat more in the rib area first, at the first rib. Gus is like this and so is Zhi (but it's easy to see on Zhi because she's hairless and built like a sighthound). Other dogs again you can find fat first between the tail and the lumbars. I've never had a dog like this, but terriers can be like this, and some mastiffs. Oh wait, Tully is like that, but I never worry much about it. He eats what he eats or he goes looking for food.
Again, I dig through Maggie's fur and locate the points of her shoulders with my thumbs, right behind where he collar sits. Now I should not be able to feel her spine even with slightly more pressure. On a dog that naturally carries very little fat, there will be a lot of hard muscle and connective tissue here that makes it feel like there's bone under one's fingers, but if you slide back and feel the vertebra, you can feel the difference in the structure of the withers. You can also do the pinch test. The same is true for the area between tail and lumbars between the pelvic points.
I'm waiting for a chance to get to the groomer to get Zhi a serious overhaul. Possibly next week sometime. If I do that, I'll post pics of her - she is slightly overweight at the moment (bad mommy! 
) and it's super easy to see the difference between her fit condition and her current chubby self.
Wait, I found a good picture.
If you look carefully, you can see hipbones (including the femoral joint), and that last rib. The withers are smooth but defined (barely visible under her crest). No vertebra are evident but you can feel them quite easily with the gentlest of pressure, with her thin skin and lack of hair. Ditto with ribs - you'd be able to feel all but possibly the last in the condition above.
I found another - my old Jen dog, after playing with the hose, soaking wet. We had a ton of trouble keeping weight on her - she turned out to have ovarian cysts and finally stopped having the life drained from her via hormones, after we got her spayed.
A bit of an awkward angle (I'm on the porch looking down, I've been spraying off the porch and she was chasing the water). You can see her vertebral structure very well, here, though.
Actually, you can't see it, which was the gold standard for me with this dog. It was quite a battle for so long. Though she's short haired, it's more of a labby coat so unless you know exactly what you are looking for, you still would have to touch her to locate pelvic structure and those lower lumbars. You can easily see waist, and the last rib. She's pushing hard on her hind legs so it's easy to see the femoral point also sticking out at a weird angle.
Here's an image from when she was still cycling (four or five times a year!), two years prior to the above picture. This is very thin but still well-muscled. Note vertebra not evident from this distance (even though the spine is under tension), and there is muscle from waist to femoral joint, not the hollowing typical of the beginnings of atrophy. My concern was less about her condition and more about the evident stress her system is under - dull coat blowing out. That was hard to explain to a vet who was simply delighted to see one less fat dog. I had to bring in one of my other dogs to show the difference in coat texture (and have him feel the difference between what I considered ideal in terms of condition, and what I wasn't comfortable with).
On my memory card which I need to transfer to my currently full hard drive, I've got some pictures of a recent "graduate" with a similar coat and build to Jen's.
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