The virtual image you see with most beamsplitters are "direct View" and don't have a ground glass. You can not preview DOF without a ground glass viewfinder. Your eye can change focus to see other parts of the image that are not in the focus plane. The ground glass creates a "real" image which allows you to judge focus. It may be possible with a lot of experience and discipline to keep your eye focus on the frame markings and judge the depth of field. I'm not familiar with specific Super 8 models. The reflex Bolex 16mm cameras have a beamsplitter and ground glass. But it is small and hard to judge anything but framing.
Depth of field is a function of image size, all other things being equal. This is widely discussed in relation to small chip video cameras and is true here. Super 8, with a small image size has very great DOF. If shallow depth of field is a goal, Super 8 would not be a good choice. (As with video I'd use longer focal lengths when possible, and make sure my subject is closer to the camera than to the background.)
Also, even with the large bright viewfinders of modern professional 35mm cameras, it is not safe to judge DOF by eye - so much more can show up when projected on a 40' screen. In most of my work in 16mm or 35mm we would always refer to depth of field tables. The ASC manual has them for 16mm and 35mm. Most ACs who have the responsibility to know what parts of an image are in focus, have a simple circular calculator that for a given focal length and focus distance shows the DOF. If I wanted extreme DOF I would use a wider angle lens and/or increase the lighting for a smaller aperture.
While you have little control of DOF you may have some piece of mind if you have a set of DOF tables. I have seen a number of online calculators (several on still camera websites) with which you may be able to construct your own custom tables.
