Hi, and thank you for contacting Ask an Expert in regards to identifying cucumber and cantaloupe seedlings.  You are certainly correct when you say they are very similar.  But as the seedlings develop and their true leaves appear the identification of them will become much easier.

Cantaloupe leaves are rough and lobed, similar to cucumber, but not as angular.  Cucumber leaves are lobed, triangular, with bristly leaves.  If you do an internet search for cantaloupe seedlings and cucumber seedlings images you will definitely see the differences.  With a bit of patience I am sure you will have them identified in no time at all.

Happy Gardening and again, thank you for contacting Cooperative Extension Ask an Expert.