The only reliable way is to stand in front of the plane and look for it yourself, and hope there is no MX or other forms of cancellation.

There was one time I was trying to guess the fin for AC15, operated by 777-200LR, which AC had only 6 of them. I searched on FR24 the night before, so that was within 12 hours, and I found one fin sitting at YYZ and another one coming in. Must be one of them, I thought. Yet when I reached boarding gate, I found a 773 was subbed in. I am not gonna mention the countless times on YYZ-YVR where the fin was changed very last minute.