If you were there prove it today by wearing your Founder's Seal emblems. You will find them at the Special Orders vendors. It could be worth it ;)
EDIT: Of course, it could make no difference at all - but it would be great on Bungie Day to celebrate by showing we've been there since the beginning.
EDIT 2: I don't think there was one for the Alpha as I did that too and don't have anything.
EDIT 3: To clear up any confusion: [b]Founder's Seal[/b] is the one for generally doing the Beta (July 17 2014), [b]Sign of the Elders[/b] for doing the Moon mission in the beta (July 26 2014) and [b]Sign of the Founders[/b] for playing in the first week of Destiny's release (September 9 2014) (http://destiny.wikia.com/wiki/Emblem).
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Got my Sign of the Elders equipped, do have Founders Seal but prefer this one.