They don't really know until the clubs and/or players confirm it officially though. The unbearably cringeworthy "here we go" shtick is actually worthless if you really want the peace of mind of literally knowing that a transfer has happened.
What I find funniest about his tweeting is when he adds "details" that are totally obviously made-up like "now the player will take 63 and a half hours to think about the offer", "Romelu didn't enjoy his beverage on the plane", "the offer is 5,710,019 lower than the selling club wants to get", "one of the doctors at the clinic where the medicals are taking place is on maternity leave", etc