4:23 PM, Sunday July 25th 2021
Hi! So while your lines look like they could definitely use some practice, it's pretty normal to have lower than usual quality lines on this exercise in particular. The lines used here are shorter than the ones used in other exercises, and you're drawing a box, that tends to give everyone (myself, back when doing this exercise, included) to over-focus on having the lines reach their endpoints instead of confidence. I do recommend practicing some ghosted lines to get them better, especially shorter ones, but high line quality is really not required at this point in the lessons.
To answer your question, the best way to distinguish the horizontal and vertical lines representing the front face of the box and the side lines representing the depth lines of the box is just that: the horizontal and vertical lines should be parallel or perpendicular to the horizon, while the depth lines should not be (and are hopefully going towards the vanishing point). Now, from your work, I can see that not all of the lines that are supposed to be horizontal or vertical are, but with a closer look I can tell pretty well which ones are meant to be horizontal and vertical, and which ones aren't.
Hope this helps!