Buying a share of property from connected person

[D Boy]

Hello,

I would like to purchase a 50 percent share of a property from a connected person. I understand that the sale must be conducted at the "open market value" of the property. Two questions:

1. Is there an official way of ascertaining this prior to the sale (e.g Council valuer, estate agents).

2. How would the IR be able to find out the open market value of a property sold in the past(in the event of an investigation), in that there would be no way to assess the condition of the property at the time?

thanks in advance.