How to check tire pressure on autofil tires w/o the tap...

As far as checking the pressure. You can pull a line off at the hub if you see what looks like a valve stem check it just like a tire. If not you’ll have to take the line off at the wheel.
You can have different pressure on a set of duals. I picked up a bolt on a outside tire and the inside didn’t lose air.

 

Just unscrew one end of the autofill line then check pressure as normal.

 

How low is the tire... Like 5 to 10 pounds low... Or like 20 to 30 pounds low??? How long has the trailer been sitting since it last moved.

 

There’s a valve under the trailer that supplies air to the system. Make sure it’s open.
Take a line off at the wheel and see if it’s the same as at the hub.