That is exactly how you tell.
Get under & check that the drive belt is in good condition and wash the tranny to make sure the oil can cool.
Check the cooling fan is still attached to the drive shaft.
After that, go look for some one who repairs trannies or go to Mulligans and get a new one.
If caught early they can be repaired cheaply, the longer t\you wait the worse they get
My test is removing them from my tipping trailer.
If I have to drag them down the slope they are good
If they roll under their own weight they are gone.
The faster they roll the worse they are