Do all dogs need their anal glands emptied? And how do you tell if they do? I know that some dogs scoot about the floor if they are full, but is there a way to tell? Can you see if they are full?
Thanks for your help in advance.

No, not all dogs need it done - most manage to do it automatically. How can you tell if they need doing? Sometimes you can't. :( One of mine developed an abscess which burst within 6 hours of it first appearing. She'd shown absolutely no signs of distress or discomfort - no licking, no nibbling, no scooting.

As JG said, some dogs dog and some don't. Our 2 and 6 year olds have NEVER needed them emptying but I have done Idõ's quite a few times since we got him.....doesn't bother me, we usually notice the smell(quite distinctive :D) first then a quick squeeze with a few pieces of tissue and its over ;)
They are all on the same, basically, food and also have chicken wings/legs at least every couple of days so who knows why some do and some don't...I see it as another little Idõ quirk :D

In my very humble opinion I have tended to find it is mainly the smaller dog who have the problem although obviously bigger ones do as well.  I actually sent my poor old Thomas down christmas eve as he had a lump in between his bum and tail which I thought was his anal gland but turned out not to be.  He sulked with me all over christmas :)  The other dog in my families dog who has ever had the problem was a westie we had about 30 years ago.  I feed on various foods over the years.