Hi,
Capital Gain Tax is applicable on sale of Capital Assets as defined u/s 2(14) of the Income Tax Act. Agricultural Land is not considered as a Capital asset if it satisfies the conditions prescribed in Clause (iii) of Section 2(14) of the Income Tax Act. So, in your case, if the Land qualifies for Exemption, then there will not be any Capital Gains.
However, if the land is treated as a Capital Asset, then there will be Capital Gain Tax. In your case, it will be Long-term Capital Gains. If you don't know the Cost and date of acquisition, same can be fetched from the revenue authorities of the concerned village where the Land is located.
Even if the Land is treated as a Capital Asset and there is a Capital Gain Tax Liability, the same can be avoided by claiming exemption u/s 54B (Buying another Agriculture Land) or u/s 54F (Buying or Constructing another Residential House) or by Buying Capital Gain Bonds u/s 54EC subject to the conditions prescribed under respective sections.