How to calculate value of separate structure at home

We have a separate structure in our back yard which includes an apartment which we rent out.

 

We are selling the entire property. How do I calculate the value of the separate structure?

 

The realtor's listing values the home then mentions the second structure as an added bonus but I can't get any clear figure from then on how much value that structure adds in the sale.

 

When I do my taxes I'm going to need to pay capital gains tax on the portion I rented out... but how do I calculate what the gain was?