where to check transmission fluid on a 2006 Navigator?

There isn't a way to check it- the ZF transmission likely installed on your truck only has a drain location and a fill location. Is your motor having shifting issues? How many miles does it have?

I imagine you want to check it because you're having issues? Listen, take your car to your local mechanic. Read this, http://www.carcomplaints.com/Lincoln/Navigator/2006/transmission /power_train-automatic_transmission.shtml

Don't worry about the transmission fluid unless you have a visible leak.

Yes IAM have shifting issues. The vehicle has 160,000+ miles on it