A lot of the 'poorly' bred silver phase Welsh will have a brown tone, or cast to them. I say 'poorly' only to mean that they're not bred to the SOP and are not culled for poor color/markings. Not to mean that they're less 'valuable' to a home flock - just those interested in show.
That duck has the gold cast to her head, I think she was gorgeous, however, I don't believe that she would be a good specimen under the SOP. Since I don't show I went ahead and kept her. She clearly shows the vibrant blue wing speculum. Gorgeous blue!
A lot of birds will also have that brown cast to their breast area - again, not a good SOP specimen but for the backyard/mutt flock not a problem. Only a problem if you're wanting to promote the breed and perpetuate it with the SOP in mind.
Gold phase birds are a light fawn, there aren't the dark tips to the feathers like with silver. I'll have to see if i have a photo of a gold hen. This drake will show what the other discernible feature is, the tail head.
This is a photo of a not-so-good gold as she doesn't really have the brown tips to her feathers like she should - but she does have the other characteristics of the gold phase in the tail head and wing speculum, which I also have a photo for. She is not a good specimen because she has the improper bill color. I did cull for that although with this duck I kept her as she was the only gold phase I had at the time. All of my golds seemed to be prone to premature death for some reason. If a Welsh was going to die or be killed, it was a gold, usually. Blah. Brown is one of my favorite colors!
Hope that helps. Those are the two characteristics that I am aware of that definitely determine whether a bird is gold or silver.