Missing Transmission Fluid Dipstick

Hello everyone, I recently bought a 2007 WRX and went to change the transmission fluid on the car today.
After removing the intercooler I found a small tube that leads to the transmission but, there is no dipstick? It just has a small orange rubber cap covering the tube.

Is this normal? How can I check that i've filled the tranny oil up to the right amount?

If it's not normal and the dummy that had the car before me thought it'd be a cool idea to replace it with a rubber cap, what is the part number for the dipstick, i've looked but only see the one for the engine oil.

I am not really a "car guy" but have been caring a lot more for cars ever since I bought this WRX.

Any help is appreciated thanks!