The estimated time for changing your CVT's transmission fluid is around 60,000Kms. There isn't a transmission fluid dip stick to check your transmission fluid level.
What you will have to do is turn to open the drain bolt and a little not all the way or your transmission fluid will just pour out within seconds. Turn to to open the drain bolt a little and check the color and the stickiness of the fluid if the color of the fluid is dark green and it's sticky, it's time for you to change and flush the transmission fluid.
The 1.5L engine that comes in the Yaris is a 1NZ-FE, it has a completely different non-CVT transmission.