The crank sensor is located on the front engine cover, and creates
pulses" of AC voltage as the 3 fingers on the crank pully spin by. The easiest access is from the drivers side. The sensor body is attached down by the crank pully, and is held on by a 5mm allen bolt. The harness is held by a few zip ties, and rthe connector is the dirty thing tied down on the hydraulic hose on the front of the engine. They fail a lot!!. You can check resistance. I think it should be around 650 ohms. Also check the inertia switch in the footwell. Look in the owners manual for info. Otherwise, try to track down the official service CD ( available from a lot of the vendors) It should have the year by year product info books and they will show the location of every fuse and relay in your car. The books can sometimes be found on ebay. You would want to find the 90 XJ6 product information guide. They were given out to dealer mechanics "back in the day" and are really good quick reference guides.
BTW, why won't it start? Do you have spark? does the fuel pump run when you crank the car? There are things it would be helpful to know to be able to provide help.