Hello,
You can interpret it as "You never know what will come out of it" pointing out to the uncertainty or ambiguity of the result.
here are some some examples about the phrase "Belli olmak":
Yağmurun ne zaman yağacağı belli olmaz, biz pencerelerimizi kapatalım.
Hayat ne gösterir belli olmaz.
Sınav sonuçları belli oldu mu?
Fırtınadan dolayı uçağın ne zaman kalkacağı belli değil.
I hope this helps.
edit: http://www.tdk.gov.tr/index.php?option=com_gts&arama=gts&guid=TDK.GTS.58ad54ae5e7f11.40621928
22 กุมภาพันธ์ 2017
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You can say these both.
You never know
You never can tell.
16 กุมภาพันธ์ 2017
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Belli is actually slang writing of belirli.
Bu belirli olmaz = This is not to be sure/certain