You can change the apparent depth of field by adjusting the aperture of the camera in the same way you control the depth of field with a real camera. Depth of field is controlled by the ratio of the distance from the camera the focus point and the aperture. The bigger the aperture the shorter the depth of field. Fortunately AE's camera is not sensitive to light so you can use as small an aperture as you'd like. An aperture value of 0 will put everything in the scene in focus.
There is however, no way of seeing the depth of field as an overlay so you have to just put objects in your 3D scene and adjust the camera position and aperture to get the results you want. AE's camera will essentially mimic a real camera so you can achieve the results you want.
Check out the Camera Lens Blur effect, which allows you to specify specific ranges in which items are in focus: