how can u tell if water has got into the transmission?
Got to ask… Why do you think it might?
Generally it means you need to drain the fluid. It can vary depending on the car and the transmission.
Tell us the make model year and transmission type (manual or automatic and which automatic if there were more than one type available.)
Is this a flood car?
You can drive the car until it is well warmed up, then examine the dipstick (if there is one). Look for bubbles of water in the oil. Or you can drain off a few ml of fluid for examination.
If is virtually impossible for water to enter the transmission from rain or from associated splashing. If you do find water in the transmission fluid, suspect a leak in the radiator and oil cooler unit.