There is no absolute proof. However there are indicators that are very compelling including the fact that it is near-impossible that practically ALL Jews would have the tradition (which was the case until the Reform movement took hold 200 years ago) that a particular event was witnessed by ALL Jews unless it actually did.
The key word above is ‘ALL’ Jews.
No other religion, out of the hundreds that exist, claims its beginnings by a mass revelation – it’s always, and without a single exception, one or a few people saying a miracle or revelation took place.
As Rabbi Keleman puts it – imagine if the Pope suddenly announced that God personally spoke to ALL Catholics at some point in history and this was revealed to him in a vision. However much respect and awe he inspires, he would soon be out of a job. Since not one Catholic has that tradition from his father or teacher, it would be clear he is lying.