For transmission Oil, dipstick (if present) can best guide about Oil condition and level.
Here is what Toyota says about vehicle with no dipstick:
My vehicle does not have a dipstick to check the transmission fluid level. How do I know if the transmission fluid level is correct and how do I check the condition of it?
_Automatic transmissions which use World Standard (WS) fluid are sealed and do not consume fluid. This eliminates the need for periodic fluid checking using a dipstick. The transmission fluid does not need to be replaced unless an inspection shows it has become necessary. _
Refer to your Owner's Manual to determine if your vehicle uses WS automatic transmission fluid.
Refer to your Warranty and Maintenance Guide to determine when a transmission fluid inspection is recommended.
If you notice a transmission fluid leak from under the vehicle, we recommend taking your vehicle to a Toyota dealership for diagnosis.
(Source: http://toyota.custhelp.com/app/answers/detail/a_id/7739/~/my-vehicle-does-not-have-a-dipstick-to-check-the-transmission-fluid-level.-how )
Following article is worth reading, if anyone would like to go in to the details:
http://www.agcoauto.com/content/news/p2_articleid/345
