Are wikileaks considered valid proof in a court of law? How does the government assess whether a leak on wikileaks is legitimate, and is the assessment made on an individual basis, if so what is the process for determining their validity, or are they all rejected or accepted?
The courts are never in such a broad position to rule on generic wikileaks evidence. Theoretically, a court could have to rule on the admissibility of a specific document, so in that sense the matter is always dealt with on a case by case basis. There is a "document" that purports to be somehow from Amazon, reporting that a certain grey building with no architectural charm in a nearby village is an Amazon data center. Legally speaking, nobody cares, but let's say that there is some legal reason why it matters whether the claim is true (I stipulate that the document passes the relevance test).
The publicly-posted document has no probative value, because anyone can create a file containing the word "Amazon". It looks like a cleaned-up OCR scan of a printout (hence the complete failure at non-English characters), and lacks any clear indicia of Amazon origin. Thus the party would have to establish that the document is authentic, and there must be a provable chain of custody from the author to the submitted document. Federal Rules of Evidence cover questions of authenticity and contents of admissible evidence in art IX, X. Nothing in rule 901 suggests that such a document would be admissible.
Very theoretically, if they happened to also have the original paper document, there might be sufficient forensic evidence to prove that the document is traceably come from the purported source. The wikileaks document per se would still be inadmissible, but the (more) original document could be admissible.