The first sign of transmission problems does not necessarily mean your vehicle needs a new transmission. It could mean one of a whole array of problems including a clogged transmission filter. Familiarising yourself with the signs of a clogged filter can possibly save you money from a dishonest mechanic who will tell you that the transmission should be replaced. There are specific symptoms of a clogged transmission filter for automatic and manual transmission vehicles.
Watch for a slipping transmission as you are driving.
Listen for a high-pitched whining or whirring noise that appears to be coming from the engine as you are driving your vehicle.
Pay attention to the car driving normally for a few minutes then slowly losing speed, but running fine after the car is turned off for five minutes.
Try moving the clutch on the vehicle when the vehicle is on and check if it has any free play in the motion.
Take your vehicle in to a licensed mechanic to adjust the clutch if it does not have any free play in the movement.
Try replacing the transmission filter if the adjustment did not improvement the clutch movement. You may need to replace the clutch if this did not improve the clutch movement.
Your vehicle may not experience all above the above signs of a clogged transmission filter, but only one of them. Replacing the transmission filter may be all that is necessary, assuming the clog is not from a deteriorating transmission.