For the 5 speed Aisin you have a dipstick so you can see how the fluid looks - for the level it's trickier to get a good reading
https://youtu.be/czDDSwguYR8
As said above, if the fluid looks completely dark brown, it may be the original one. It is not a good idea to have the garage flush it all at once. Plus, you need to make sure they use a correct 3309 norm fluid - in Europe the german Mannol should be a choice, or Toyota Tyoe IV. If it was me, I would only replace about 2 liters at the return hose, then drive the car inside the city for a few weeks to allow the transmission to shift multiple times - this gets it to adapt faster. Replacing too much fluid then driving the car or the motorway long distances may cause clutch plates wear. As for resetting the transmission, there are two resets: the fluid counter reset and the transmission adaptations reset - the second one must be done only when replacing transmission parts, the first one when replacing the entire fluid