Testing for a bad MOSFET
Connect drain to source and measure the resistance to the gate. it should be infinite (much more than 1 Meg) with both meter polarities.

Connect gate to source and measure source measure to the drain. 
it should look like a diode 

The data-sheet says up-to 1μA leakage with the gate connected to source depending on your meter that could be the 500kΩ reading you are seeing.

Logic level MOSFETs don't turn off as hard (with Vgs=0) as regular MOSFETs do due to the threshold being closer to 0V