In the parks, at most of the CS and food cart places, the menu board either has the snack/dining plan symbols posted OR a sign that says the cart doesn't accept dining plan credits. In the resorts and the stores, however, there generally are NOT signs with the symbols and you have to ask. Hope that helps.
I just found this post and it was extremely helpful. http://www.disboards.com/showthread.php?t=1683477 I hope to print it out at work (boss' permission, of course ) so I can take it with me (will wait til week we leave to print out so I am sure it is updated).
I found a symbol that is displayed at locations where DDP is accepted as far as I know:
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