You could simply deduct that by reason. If you want to proof something didn't exist before a certain date nowadays you would name something like the hash of a block. You can be pretty sure if you see the correct hash of block 701000 mentioned in a message that message couldn't have been constructed before the block was mined. Satoshi proofed this by quoting the headline of a newspaper. So this genesis block couldn't be mined before the newspaper came out.

A quick google search for "Satoshi pre-mine" turned on some results like https://danhedl.medium.com/bitcoins-distribution-was-fair-e2ef7bbbc892. But then again I don't know the definition you have for relevant source