how to know if my home is under negative pressure?
In short, you want to use or rent a manometer. 



I suggest reading this article ( http://contractingbusiness.com/archive/how-measure-building-pressures ) as it details the differences in levels room to room and what the ideal pressure should be in vented and non vented areas, ie kitchen/bathroom as opposed to bed/living room.

As for the dust under the baseboards, that can be due to improper cleaing during construction of the dwelling.