This basic principle aplies to most (commercial) trucks- Freightliner, International, Volvo, Kenworth, Peterbuilt
Before beginning make sure the vehicle is parked on flat ground.
Looking at the diff housing you will find a plug approximately half way up. They come in all kinds of shapes and sizes the most common is a 1/2 inch drive. the oil level should be inline with the bottom of the plug hole. A little over or a little over is fine.
Checking the gearbox oil level is very similar, the plug is usually located on the side a 1/4 to a 1/2 way up and again can come in many different shapes and sizes.
If the oil trickles out when you remove the plug this is fine. You may have to dip your finger in, if you can feel the oil just below the hole this is also fine