You won't. What you see in preview is the last frame your cursor was in. When you drag on the timeline you move away from the cursor, so it can't be updated in real time in the preview window. Of course you can see a range of the timeline if you select that portion of the timeline, but you won't see real time changes as your drag while trimming in the preview window. You do see frames change in the timeline itself as your drag. By zooming it you can get very precise editing, down to a single frame or fraction there of or you can use the trimmer.

When trimming from the timeline you are draggin the end of an event, so the last frame shown (on main timeline) changes to reflect where you are. To see the current frame in the preview window you would need to move the cursor accordingly.

By clicking on the little square at the front of any track you can greatly increase the size of the frames. If still not enough, once expanded zoom in by using the up arrow on your keyboard.