When I started growing my own cucumbers and zucchini, I found that the seedlings that sprouted looked almost exactly alike. Luckily, I kept track of what we planted in our starters, otherwise I would have never been able to tell the difference.
Even when the plants matured a little it was still quite a challenge determining which was a cucumber plant and which was a zucchini plant. The leaf shapes are very similar and they both produce tiny needle-like thorns on the stems of the leaves. Good news is that once they started flowering, the difference was crystal clear.
Even with all of their similarities, cucumbers and zucchini’s have tell-tale ways to tell them apart. The coolest part of it all was watching them grow into way different plants than I had originally expected. The almost identical seedlings led me to believe they were going to be very difficult to differentiate, but once they hit a certain stage of maturity, I realized they would be very different despite what I originally believed.