Hi Roger,
A number of things that to me are indicators, but that the fieldguides don't portray, would be:
1. Song tends to be more secretive, feeding under hedges or trees, whereas Mistle are much bolder, strutting around on open grass
2. Mistle stands in a more "cocky" way, breast out, head up, whereas Song seems to often have its head down, furtling around turning leaves over, etc
3. On general shape, Mistle seems large-bodied with a small head, whilst Song always looks better-proportioned, with a smaller rounder body and larger head
4. As David says, song always looks brown, almost golden brown on the upperparts, whereas Mistle looks grey-brown.
5. In flight, Mistle shows white corners to the tail as well as the striking white underwings
6. Call: Mistle does a loud rattle, often as it flies, unlike Song's quiet psssiip
7.The breast spots on Song are more teardrop shaped, on Mistle, flatter and wider.
8. There is more of a golden wash on the breast and flanks, whilst Mistle has a very white-looking breast
These are just a few from memory - hope they are of use
All the best
Sean