Is there any equivalent for this Persian expression "____ is like an unopened watermelon"?

Solution 1:

A crapshoot is a way of saying that a particular chance being taken will have an arbitrary result, and is slightly opinionated in that it suggests a "good" result is (somewhat) less likely, or at least is chancy (like anything random). Marriage is a crapshoot. You have to roll the dice. Note that the reference 'crapshoot' is to 'shooting craps', a dice game. (Thanks to @Sabre for underscoring this.)

The proof is in the pudding is an opinionated way of suggesting that only by sampling a thing will you know if it is good. (The expressed opinion is that it is, indeed, quality.) Is this the right marriage for me? The proof will be in the pudding. The older, more long-form way of saying this is 'The proof of the pudding is in the eating.', but you will often hear it shortened. (Thanks to Paul du Bois for pointing this out.)

Anyone's guess is a more neutral way of saying the result of a choice is uncertain. Whether the marriage will work out is anyone's guess.

Solution 2:

Forrest Gump's mama said:

"Life is like a box of chocolates. You never know what you're gonna get."