How can a reader decide if a script idea is original or not? I mean there are so many movies out there, one just cannot know all of them.

A great point and yes, it's absolutely true a script reader cannot possibly have watched ALL movies or TV shows (that said, it annoys the crap out of me when someone wants to work in TV or Film - including screenwriters! - and it's clear they haven't watched even HALF of the latest releases, or the classics; or they DON'T have a niche interest - ie. creature features; J-Horror inspired movies; representations of women etc - The short version is: be PASSIONATE or WHAT'S THE POINT! Back to your Q ;).
So instead then, I think it's key to think of originality as being "cyclical" - we see LOTS of the same idea all at once (and for various reasons), both produced and spec ... On this basis, something will jump out at us when it's DIFFERENT to the rest in the pile. Sometimes it will be obvious why, like the latest interest in female protagonists is a concerted kick back against the tired hero's journey; other times it will not be so obvious, but nearly always it will create a race for "the same, but different" (if it's successful) ... Until everybody gets sick of it and then the search will be on again for "original".

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