The first bit translated as "how do you know it's a foke song..." (but it should be "Sut ydych chi'n gwbod bod e'n gan werin...")
The second bit is less clear: "a ydi e pob tro" (which should be "a ydi e bob tro") means "and is it always" (or, more literally "is it... every time"). But I'm not very sure about foddol. It would have to be a mutated form of either boddol or moddol. The second is a word, which means "modal", but I just can't see how that makes sense here. Boddol is not a word I've ever heard, but I suppose it could be meant to be "boddhaol", which means "satisfactory".
So that would leave us with "How do you know it's a foke song, and is it always satisfactory/modal?"
Please let me know where this comes from and what it turns out to have been supposed to mean!
I'm sorry, but "if everyone is agreeable" and "has it always existed" are not good translations of the second half. If it was clear what was meant by "foddol", then this would be a lot simpler. As moddol just doesn't seem to fit the sense, it looks like a mistake to me (especially considering the other minor oddities)