Estimating national intangible wealth
The reference is World Bank (2006). You can see the detail of the methodology in Appendix 1, and their estimates of intangible wealth in Appendix 2. 

There are other methodologies to measure intangible wealth (e.g. here and here). Alternatively, there are many ways to compute natural and physical capital, which will give different results to the one given by the WB, even if using the same formula to compute intangible capital. For example, see here. If you search in google or here things like "measuring capital stock", you get tons of hits too. 

I think that because this is a relatively new research area, there is no "official" guideline about how to measure intangible wealth. Maybe in the future some sort of standard will be produces, pretty much like the standard for GDP came to be (which is still subject to criticism anyway).