Yes, in most cases you will know well before check in, but how fast the news reaches you depends on the supplier. When a hotel or a supplier cancels or rejects a reservation that was already confirmed, that change comes back through the connection and the status on the booking in your portal is updated, normally from confirmed to cancelled. Your portal can also flag it for you so you are not relying on noticing it by chance, and the original booking record stays in place so you can see what was held and what was paid.
Where we have to be honest is the timing and the detail. Some suppliers send the update almost straight away, others only pass it on when their own staff have processed it, which can mean several hours or longer. A few send the new status with no reason attached, so you may know the room is gone without knowing why. And if a property has spoken to your customer directly rather than going back through the booking channel, nothing comes through the connection at all until somebody tells you, which is simply outside what any portal can see.
Because of that, treat the status in your portal as your main record but not your only safety net. Bookings that came through as on request, long stays, group bookings and anything over a busy local event are the ones most worth reconfirming with the supplier a few days before arrival, and it helps to keep a note on the booking of who you spoke to and when.
If this matters a lot for the kind of travel you sell, check with us which of your connected suppliers send live status updates and which do not, and set up alerts on your account so cancelled bookings reach you rather than waiting to be found.