You should be able to check your glow plugs with an Ohmmeter without having to remove anything but the connector that goes to the Glow Plug wires at the. (The Diesel Giant site has a trouble shooting pictorial for 617 engines but enough of it is close enough to be helpful.)
Your Glow Plug Relay may look different but the connector on the left of the pic is for the Glow Plug Wires and they are numbered per cylinder.