I was looking into creating a wizard that remains hidden for the most part from combat (potentially even to go as far as to hide in a fellow party members' backpack, they are a halfling) and thought about this question when it came to using the senses of my familiar as created by Find Familiar.
While your familiar is within 100 feet of you, you can communicate with it telepathically. Additionally, as an action, you can see through your familiar’s eyes and hear what it hears until the start of your next turn, gaining the benefits of any special senses that the familiar has. During this time, you are deaf and blind with regard to your own senses.
So looking through the senses of my familiar, in this instance let's say it was a Raven and I can see the battlefield through his senses as he flies about 20ft above us providing an aerial view.
Casting Time: 1 reaction, which you take when you see a creature within 60 feet of you casting a spell
You attempt to interrupt a creature in the process of casting a spell. If the creature is casting a spell of 3rd level or lower, its spell fails and has no effect. If it is casting a spell of 4th level or higher, make an ability check using your spellcasting ability. The DC equals 10 + the spell’s level. On a success, the creature’s spell fails and has no effect.
Following the earlier example let's go even further and say that I can see the battlefield from inside the fighters' backpack (hidden completely) and someone, let's say a sorcerer within a 60ft range of the two of us decides to try and cast a spell, since I am in the backpack hidden away I usually would not be able to see out of it, except in this scenario I am using my familiars's senses to see and I can see the enemy sorcerer and the fighter on the battlefield.
Would I then be able to cast counterspell to stop the sorcerer?