With respect to gifts, you generally need to know the donor's adjusted basis and the fair market value on the date of the gift.

Since the land was purchased circa 100 years ago, you would almost certainly use the cost basis to calculate gain. However, you at least need to know the approximate date the land was purchased for the purposes of researching the cost of the land at that time (an appraiser could most likely value the land, but would need to know at least a date range).

If you cannot determine the purchase date (or price paid), then you are relegated to using a zero basis which, unless the land is extremely valuable, probably is not going to make a huge difference (considering what property was selling for ~100 years ago).