Eyes, ears, contacts.
Like if your only job was to find good footballers, over time you would build a decent network of all 3.
Then, if you are acting for a PL club, doors will open.
I reckon anyrate.
..statistics play a big part I reckon. Clubs have analysts these days looking at the stats that matter for each player in each position. This either confirms what their eyes tell them or points them in the direction of players to watch.