Answer: Well, this is a tough one. The leaves look the same, the flowers look the same. I think the only way you are going to be able to tell is by watching them. If you planted small pickling varieties keep an eye on all the cukes, the small varieties will stay on the small side and not grow long like the slicers. Small picklers usually get to be about 4-5" long, slicers between 8-14" long.
Here are some general differences--though these may not be true for all varieties: Picklers tend to have thinner, more tender skin, and often produce their crop within a week or two. Slicers have thicker skins and may continue to set fruit for 4 to 6 weeks.
And, really, it doesn't matter...you can pickle a slicer (harvested when small), and slice a pickler.
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