You can't. You have to test it.

 

The easiest way to test it is to swap it with another relay. On my 92xjs, the horn relay is the same type of relay and is located in a small bank under the hood over the passenger side front wheel. I think that there was one or two screws holding a small cover pver this bank of relays which made it very easy to access.

 

In my RHD 1990 XJ-S, the fuse cover does not list a fuse for the brakes. I have a third light. I was changing the bulbs to LEDs.

Park lights still work great, but no brake lights from any of the three fittings. I have replaced LEDs back to the bulbs.

Any idea where the fuse is? It is not listed in the manual either?

 

It is fuse no. 5 in the main fuse box, which controls direction indicators, stop lamps, and kick down switch, according to my XJS HE Jaguar OEM book. 15 amp. If your indicators are out too, fuse looks likely.

Greg