Ive noticed my fuel mileage has been steadily decreasing, and I hear a tick like a slight exhaust leak / ping while driving. I check my oil every time I stop for fuel, and it seems to have been slightly increasing every stop.

What can I check before taking it to the dealer?

 

if its under warranty, check nothing. let them deal with it.

 

change fuel filter and call tow truck one thing you can do is pull th edipstick and drip a few drops of oil onto a paper towel If they is diesel in the oil you will see a light ring around the oil drop