Have it looked at by a jeweller.
I agree with the first answer, also you could look for the hall mark (authenticity stamp) on the inside of the ring.
First Off Does It Have Diamons In It? If So Look At The Back Of Them And If There Are Holes There Then They Are Real. Diamonds Need To Breath, Sounds Weird But True. Then If It Dont Have Diamonds Then Take It To A Jeweller, That’s The Best If It Dont Have Diamonds.