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Each side has the opportunity to use their own experts to ask those questions and analyze the forensic integrity of the evidence at issue. Even if your side doesn't have an expert, your attorney still has the chance to question the other side's expert.

So if there's a piece of evidence based on an email sent from Alice to Bob, the way the evidence gets introduced is that it gets authenticated, by someone who would be in a position to speak to whether a particular document is authentic. The other side can seek to exclude the evidence if the basis for authentication isn't strong enough. Or, it comes in, and the other side might want to challenge that the document actually represents what the other side wants to prove: maybe casting doubt on whether other people had access to Alice's account, etc.

Or if you want to use a surveillance camera video, you'd generally have someone who maintains the system testify as to how the system records, where it stores the data, and how it adds timestamps to different videos. Then that technical person can usually testify that the timestamp is accurate, etc., and might have to answer questions about what happens when the system loses power or a connection, etc.

So it's not that the courts in the US actually test the validity of evidence. It's that the parties involved in the case can challenge the validity if the circumstances call for it.