The value of an asset generally is determined by depreciating the cost of the asset over its useful life. This is done incrementally so that, at any given time, the value of the asset is reflected in accounting journals. Land, however, is an asset that is never depreciable, meaning that it doesn't lose value with time like a building or a car does. As such, it's necessary to determine the value of land through other means.
Determine the ratio of land value to building and improvements value for the rental property at the time of purchase. This often is done using an appraiser.
Subtract the portion of the original purchase price attributed to the value of the building and improvements from the total purchase price. This determines the value of the land at the time of purchase.
Research the selling price of similar rental properties in the area. This indicates the fair market value of the property.
Apply the ratio of land value to building and improvements value to the estimated fair market value of the property. This approximates the land value of the rental property at any given time.
Hire a public or private tax assessor to appraise the value of the land. This is generally required on a regular basis by governing bodies that levy property taxes.