How do I know if my house's foundation can bear an additional story on top?

It is not that your foundation bearing the load of the additional story on top of it but the soil underneath your building. 

The footing of the foundation counters the downward acting force of your building with the upward supporting of the hard soil. 

When you are building something on the soil, you need to check the soil condition to determine your foundation system. 

A typically cut or filled land would give you a different soil condition to determine the type of foundation system to be used. 

A filled land usually consists of soft soil on the top layer and hence, the foundation system needs to reach the hard layer soil further below from the surface of the ground. This is why certain two-story houses will need to have a piled foundation while some do not. 

The size of your footing is also the main factor to consider the amount of the downward acting force it can counter. 

To find out if your house's foundation can bear another story on top, you can:- 

1. Consult a Geotech engineer for your soil condition. Test like Mackintosh probe test can be carried out to investigate the soil condition. 

2. Check your house plan for the size and type of the foundation. 

3. Consult a structural engineer for the foundation design with the soil test result. 

Methods like an assumption or simply observing neighbor buildings to determine the soil condition of your own plot of land are not encouraged as it will put you and your building on a high risk such as settlement, hairline cracks on the wall, structure and worst, structural failure.