Sorry, somehow I missed your response - not sure how.

If you can get the case open (see the FAQ), then you need to set the meter to a range that can handle 60V DC. Probe the neg lead to the case, and the pos lead to the plus lead of the electrolytic capacitor next to the inductor on the back of the display board. This is also marked on the board as TP1. Careful you don't short against anything. I posted a photo of this recently somewhere which I will have to look up for you.

You should get a voltage between 48-56V DC. If not, this either means that the local PSU on the display board has packed up, or that the display board is not being fed from the main board.

You can find the photo here