It really doesn't matter, and is not something that is possible to prove. Like a Buddha image, a pagoda, or any sacred object, it is just a skilful means to direct your mind towards the noble qualities of the Buddha, Dhamma, and Sangha. If you have doubts as to a relic's authenticity, your mind may waver, and you may be disinclined to worship at a shrine. However, if you understand about the qualities of the triple gem, the relic is not the important thing — the quality of devotion to virtuous qualities is what matters.
Don't be a superstitious follower of the Buddha who relies on sacred objects. Read the Candala Sutta
and reflect on its meaning.
On the BBC today I saw an item about the theft of Rhino horn from museums. Apparently it is about twice the price of gold. Superstitious people believe it has great potency as a medicine. In fact, it is just made from Kerotin, the same substance in hair and finger nails. It would be just as effective to use toenail clippings in medicine as to use Rhino horn. If only we could sell our toenail clippings for £2,000 an ounce!