Differentiating a wolf from a fox is definitely a bit tricky. I struggle with it myself sometimes, so I might not be the best person to seek advice from, but here goes—
Taking images of foxes and wolves and blocking out the core facial shapes can be very useful.
You’ll quickly notice that, typically, a fox’s features can be likened to triangular shapes— whereas a wolf is more blocky and square.
The slope from the forehead to the nose is much more subtle on a fox, while it’s sharper and more pronounced on a wolf.
The ears of a wolf are usually smaller than a fox’s in proportion to the head. While in reverse, the actual nose of a fox is smaller.
Pay attention to all of the subtle differences you’ve blocked out for either species and don’t be afraid to exaggerate on them. When I draw a fox, I tend to make the nose longer and sharper. I also make the lower jaw less visible, as it creates a more pointed shape. I do the opposite with my wolves so their snout is more boxy and rectangular, rather than triangular.
I find that the cheek and neck fur are also very useful. Foxes can certainly be fluffy (particularly their tail), but the neck of a wolf has overall more volume than a fox’s. If you show the neck fur sweeping into a triangular shape for a fox, it maintains that sleek feeling as the face transitions into the body. Blocking out and exaggerating the fluff on a wolf, on the other hand, creates a “fuller” look to the face and can make the creature seem larger and bulkier if carried throughout the rest of the neck and shoulders nicely.
I could go more into body shapes, but I don’t feel super qualified to do so. My advice would just be to keep these words in mind:
Fox: sleek, streamlined, triangular
Wolf: powerful, fluffy, boxy
I would also recommend referencing whenever you feel you need to. To avoid copying from references directly, grab 2-5 references of the same subject in various poses and angles so you’re forced to pay more attention to the subject, rather than a strict image reference.
On another note, I have some canines tutorials in this list.
Hope this helps!