A real estate agent can easily look up comparable sales during that time. Unfortunately, the time to ask would have been as you were hiring him/her to sell the house, a bit of extra work for their cut. If no agent was used, I'd find one who was willing to charge you a nominal fee to do this. Sales records for 2012, only 6 years ago, are easily found.
Did the heir leave the house to a further beneficiary? In that case the 2012 value is pretty moot, as the value is stepped up again on that most recent death.
Update to respond to comment.
When I die and the house is transferred on my death, it immediately (or up to 6 months later) gets a stepped up basis. The price I paid for it no longer matters. If it's sold by my daughter within that 6 months, there is no capital gain tax due. If it's sold after, but still soon, it would be a rare event that would create a gain. e.g. The house is worth $300K when I die, and a year later it sells for $310. The cost of the sale, the real estate agent, lawyer, etc, is likely to exceed the $10K it gained from the time it was valued.
The bottom line is that 2012 no longer is part of the math, the death and inheritance occurred in 2017.