My oil pressure gauge wire worked it's way loose. I found the female spade connector that goes on top of the "bell" shaped sender was also intermittent. I ended up putting a new female end on it. and that took care of it. To test the gauge/wiring you can take the wire and ground it. That should peg the gauge. You could also compare the resistance of the top of the sender and ground with the engine running. I forget what the exact range should be but if it reads super high/OL/Open then the sender is not working right.
For the fuel sending unit the most common problem is the ground strap missing/broken. It should go from the unit to the frame. The unit is electrically isolated from the tank itself for safety reasons. You should be able to access the sender from under the car on the driver's side. You'll need to remove the heat shield to get to it. You can ground the plug and see if the fuel gauge pegs. If it does, and the ground strap is good, but it still doesn't work, then most likely the sending unit has passed. Typically the float has cracked/corroded and is taking on fuel so it no longer floats. You can get a replacement but you'll want to save some parts from the original as they fit better (you can find threads on this)
