The first and most obvious visual cue that may indicate a bad transmission is leaking fluid.
Automatic transmission fluid (ATF) lubricates components to reduce friction and prevent the transmission from overheating. It also acts as a hydraulic fluid to allow gear engagement. Low fluid can lead to overheating, which is the fastest way for a car's transmission to fail. If you spot a pool of red fluid underneath your vehicle, get the leak fixed immediately. However, if the fluid appears dark brown or black, it could be burnt, in which case, a minor repair will not remedy the internal failure that has already occurred. Have your vehicle examined by a professional to diagnose the problem.
Your Check Engine Light is another obvious red flag indicating that something is wrong. If this warning light pops up on your dashboard, do not ignore it. The service light responds to sensors inside the vehicle, detecting slight vibrations that you would not necessarily notice while driving.