Suspected headlight relay problem?

I suspect my headlight relay fuse (that controls both headlights) has gone out because suddenly my headlights (dipped or main beam) have stopped working at the same time. I have located the fuse box where the relays are, and I currently have 4 relay fuses there in 4 of the 6 possible slots but I don’t know which one controls the headlights as I can’t find the manual. What sort of voltage fuse do I need to get to replace it and how do I find out which relay fuse that’s faulty and needs replacing?
Any advice most appreciated, thank you.