Hi, Shilun. What type of folder are you taking apart? I have a Bike Friday and a Dahon Speed 8 and I've taken both apart because I like to add my own components. Other than the handleposts, most folding bikes assemble like any other bike. The handlepost/stem are usually a one piece unit that can be removed with a 5mm or 6mm hex wrench. Once removed, you can remove or inspect the headset.
I really don't think you'll have any problems with either a Bike Friday or Dahon. I cannot comment on other folding bikes but is suspect they are much the same