Selling property abroad

[Jairo Montezuma]

I own 25% of a land property that was given to me (sold at a very low price for documentation purposes), but I did not pay a dime. This land is now being sold by my family that I will be able to claim 25% of its value. I don't know that the fair market value was at the time of the transfer. What would be the best way to calculate or estimate the FMV to pay IRS for capital gain tax? -- I am aware that 1031 exchange would help to avoid this tax, but I am not interested to keep properties overseas.