As has been said previously, go onto Autotrader to find the model nearest yours, with the nearest mileage and closest condition. If you can find than one so much the better. Only obtain comparisons from private sellers - it's pointless getting comparisons from dealers because their price includes a profit margin and your insurance will not cover this element.
The measure used is the current market value at the time of the loss, in other words the true measure of indemnity is to place you in the same position post-loss as you were in pre-loss, but this is difficult with cars because no two are the same.
The value you told the insurance company when you took out the policy is irrelevant - you are not insured for this amount, plus most people think their car is worth more than it actually is.