There are different methods to find out the net worth of wealthy people. Some of those methods have been explained below:
1. By simply asking: Some rich people disclose their wealth to the magazines voluntarily.
2. By evaluating public assets: If somebody has properties whose value is known publicly, it is fairly simple to evaluate their worth. For instance, Peter Thiel's stake in Facebook can be evaluated by checking the value of Facebook's stock. The value of his share gives a clear idea about how a part of his net worth.
3. By evaluating private assets: If a peron is a real estate mogul, most of his wealth would be entrenched as private assets. To evaluate their wealth, Forbes approximates the value similar properties. The process is not exact. but it definitely gives a coherent idea about a person's wealth