Size, and sometimes location. A squirrel nest is typically pretty good sized, and appears to be loosely constructed of twigs, strips of bark, and lots of dead leaves. I usually see them pretty high up in a good sized tree. Google 'squirrel nests' and you can see what the typical tree squirrel nest looks like.

Robins, by the way, build a very solid and heavy nest of small twigs and other material, packed with sticky soil or clay. These nests are usually lined with finer, softer material. They last FOREVER! You'll find robins' nests pretty low in a tree or shrub....often where you can peek at the babies face to face. Robins will build their nests where they shouldn't too! I've had them raise families on window sills (that was fun), and on my porch light (dumb bird).

Here is a link that might be useful: Robin nests

We have a small town newspaper that comes out a few times a week. There was a small piece in yesterdays paper about 4 squirrels stuck in someones chain link fence. Seems squirrels sometimes use pine needles for their nests and with the needles comes the sap. When the squirrels are tiny, they get the sap all over them, mostly their tails. The more they try to move around in the nest, the more their tails become tangled. That's what happened to the 4 squirrels stuck in the fence. Their tails were all stuck to the others. A lady got them unstuck...estimating it took over a half hour to do so. The tails of the squirrels around us are all funky, lots of the hair missing and maybe half the tail missing on some of them. Now I wonder if this is why. There's a huge pine tree behind a house across the street...?? I don't know, but it makes sense after reading that!

Squirrel nests are huge! And really high up in a tree. At least the ones I've seen. It's fun watching them run across the back porch, jump down to the ground and run to the neighbors tree and climb all the way up...then disappear into the nest. It seems generation after generation use the same nest too!