An automatic transmission rarely fails all at once. The gear change is where the first signs appear, and those signs are easy to explain away as normal driving until a shift lands hard or the car fails to hold the gear you selected. Learning the early pattern lets you write down what you see while the change is still small.
What to watch for
Shifts that come later than they used to, or that feel soft, flared, or harsh. Engine speed rising between gears without the car gaining speed. A gear that seems to slip under load, such as climbing a hill or carrying weight. A new whine, clunk, or shudder that follows a shift, or a warning light that comes and goes.
What to write down each time
Note the day, the mileage, the outside temperature, the speed and gear when the change happened, and what you felt or heard. A small notebook kept in the glovebox is enough. Notes that repeat across several trips show a pattern, while a single odd shift may not, so keep the notes for the car as a whole rather than for one incident.
What to do with the notes
Bring them to a licensed technician before the repair is booked, so the diagnosis starts from what you observed. If you hold a service contract, read its breakdown reporting clause, because some contracts require a prompt report, and a written record makes that step easier to meet. Follow the service intervals and fluid specifications named in your owner's manual. Do not keep driving a car that slips badly in traffic, and ask the technician what is safe to do until the diagnosis is complete.