I was told that I mainly use biceps to perform bent over barbell rows. However, I feel like I'm using my back. How can I find out to which extent are hands absorbing the load?
If you check out EXRX's muscle activation list on the bent-over barbell row, you'll note that the dozen or so muscles that make up the bicep and forearm are syngerists and dynamic stabilizers: not prime movers.
So while your arms are certainly involved in the lift, they are hardly the prime movers.
Even if you flip to an underhand grip on your barbell bent-over row, which will put more emphasis on your lats, your back reminds the prime mover although I'd imagine there's more loading of the biceps.
And you can prove it yourself: you can normally barbell row much more than you can curl. Rowing 180lbs for 5RM is a typical weight to see, but I highly doubt that person will be curling 90lbs 5RM for the same reps.