Consult a real estate agent or a licensed appraiser. An appraiser can use historical sales records to determine a retroactive value.
Vacant land may be difficult to value because so much is based on speculative future uses. Farmland is often quite easy, there will be reliable values for cost per acre of productive farmland. Other situations may be in between.
The key point is to make a diligent good faith effort to establish a value. If you are unlucky enough to get audited, the IRS won't give you any basis that you can't back up with pretty good proof.
