These two use the exact same mouth position. Teeth and lips sit in identical places for both, and nothing about them is visible. Only one switch separates them.
Identical Mouths, One Switch
There is genuinely nothing to see here. Both sounds put the top teeth on the bottom lip and push air through the gap. Everything visible is the same.
v in van
The vocal cords vibrate while the air passes, adding a low buzz underneath the friction. The sound has a hum inside it.
f in fan
The vocal cords stay still. What you get is pure air noise through the gap, with nothing underneath it — quieter and thinner.
The Switch Your Language Never Used
Korean's sound inventory skips /v/ entirely, which removes the voiced half of this pair before the question is ever asked. Because the mouth position is identical for both sounds, there is no visible or tactile difference to fall back on in someone else's speech. That leaves voicing as the only signal, and voicing is exactly the feature that gets flattened when a language has no use for it. Touch is the way back in, because it reports on your own mouth rather than someone else's.
Put a Finger on Your Throat
This is one of the few English contrasts you can verify by touch. Rest a finger lightly on the front of your throat and say a long vvvv. You will feel a clear vibration under your fingertip. Now say a long ffff with the same mouth position. The vibration disappears completely while everything else stays where it was. That is the entire distinction, isolated. Because you can feel it on yourself, you can build the category from the production side even before your ear reliably catches it in other people's speech — which is a useful way in when a sound is missing from your first language.