The most common sign that your cars transmission is low on transmission fluid is shifting problems where you press the gas pedal to accelerate the vehicle but the RPM goes up and the transmission takes too long to shift gears.
Your transmission should shift gears within a few seconds and if not then your transmission fluid is likely low or you have transmission problem.
Other signs that you're low on transmission fluid is Grinding or Strange Noises coming from the transmission, slipping transmission, slipping gears etc.
If your vehicle is taking too long to shift up and pick up speed then check your transmission fluid to be sure the transmission fluid is at the proper level and not too low or overfilled.
Also if you have contaminated or burnt transmission fluid then that too can cause transmission shifting problems as well as possible transmission damage.
The fluid in the transmission should be a good dark red, translucent color and if any other color it could be contaminated.