>> My St Bernard has a small flash of white on her face (rather than a wide area of white mask) with black spots fading into black, does this mean she has a showing fault? <<
St B's do not have a white mask they have a white blaze and a white muzzle. Not sure I understand 'black spots fading into black' if you meant white then I would say that is not a fault. Are you showing the dog then, if you are then you will soon find out if it is to standard, or are you thinking of breeding, if that is what you plan I think you will need to find out a lot more about the breed and I would suggest you start by joining a breed club and attending lots of events to see good quality dogs and ask for an opinion on yours. If you are just curious then you may find reading the standard some help but it takes some knowledge of the breed to understand how all the different parts described come together.