One sure sign that you're low on transmission fluid is that it takes longer for the transmission to shift the gears properly.
If you step on the gas pedal to accelerate and the transmission is not shifting up but the engines RPM is going up then you could be low on the transmission fluid.
The transmission should shift on up within a few seconds of accelerating and if not you either have low transmission fluid, bad transmission fluid, clogged transmission filter or the transmission is going bad.
Check the transmission fluid and if you're losing transmission fluid then it could be leaking out the front transmission seal, rear transmission seal or the transmission pan.
You should not be losing any transmission fluid.
Other signs of low transmission fluid are gear slippage, transmission slippage, transmission overheating, Surging Transmission or Erratic Shifting.
If you keep driving your vehicle with low transmission fluid then it can cause wear on the bands inside the transmission due to increased friction which can then lead to transmission failure where you'll need to have the transmission rebuilt or replaced.