Is there any way to tell from the outside of a peach if it's a cling or a freestone variety?
Not as far as I know, but grocery stores tend to carry freestones, whereas farmers markets will carry both.
Having purchased 90% of my peaches from the a wide variety of farmers at various NYC Farmer's Markets, I've never once ended up with freestones. Might there be a regional aspect to the answer? Maybe freestone don't grow well in the NYC region but are more popular, say, in Georgia?
From my friend who grows peaches for a living: Unless you know the variety, you cannot just look at a peach and tell for sure if it is freestone. You could make a pretty good guess, but you can't say for sure.
Having been raised on a peach orchard, the earlier varieties that ripen in mid June and July and mid-August are eating peaches, not canning peaches. They are not free stones. The later peaches, which are used for canning are freestones.