As Chuck says it's marked on your property deeds...........as I face my house I am resposible for the left hand boundaries, and partly responsible, with 2 others, for the fence at the bottom of my garden.
Oh, and at my late parents house, we kids were always informed not to sit on this particular wall because it belonged to the neighbour. Now there are new owners in there they deny any responsibility for the wall that is now in serious disrepair. Seems things are not ours when they are ok, but suddenly become so when they need maintenence.