The Persian expression/ simile "____ is like an unopened (=uncut) watermelon" implies that you never know what the given (risky) issue/ choice will turn out to be until you proceed to experience or try it.
We use it specially for speaking about cases in which we have made a (risky) choice, picked up someone or something but we are not still sure if it has been a right choice; or just for speaking generally about making such choices with unknown consequences.
Examples:
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Marriage is like an unopened watermelon: This is the most famous usage of this expression and implies that you cannot be sure if you have picked up the right person to marry. Just like when you pick up a watermelon, you'll notice if it's a good one (i.e. ripe and sweet) after cutting and eating it, you'll only notice if you have married 'The one' after experiencing the married life with him/ her (i.e. living under the same roof for some time).
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Brexit is like an unopened watermelon: That means it is still not clear what will happen after Britain leaves the EU (i.e. if it will end up in a better or a worse condition).
Is there any idiom, expression or even a simile as the equivalent for this Persian expression in English (i.e. you won't know what will happen until you (proceed to) experience or try the given (risky) issue or choice)?
Unfortunately I couldn't find anything in my searches.
Best Answer
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.