Culhwch: "fer" as a pronunciation for "far" is usually in a context such as the following: "Go down t' rud a fer piece" for "Go down the road for a far distance" -- though here it may also be "fair" as well as "far"; "Fer 's I know" -- "As far as I know", etc.; so it's usually obvious from context.
Best ways, Dusty ... study the writers who use dialect, and pick out what sounds right... but try reading them aloud to capture the flavor; sight-reading them it may sound very oddly in your head, but pronounced, they often make perfectly good sense. Such writers as Scott, Buchan, Burns, etc., for Scots dialect, for instance, or some of the Southern Gothic writers for southern accents; David Grubb for West Virginia, etc. In doing dialects, though, it's best to stick with those you yourself have heard and taken note of. There are still people who use the old East Side or Brooklyn accents in writing about some parts of New York, and those have almost entirely died out (or altered beyond recognition) now. Remember, "dialects" in writing are essentially attempts to phonetically capture the speech of a particular region (or, in some cases, where you have a strongly stratified society, denote differences in class); so there's no hard-and-fast rule on them; the point is to listen very closely, and do the best you can to put on paper the sound you hear when they speak aloud.
Also... though I like dialect myself, and find it adds tremendously to the feel of a place in a tale, it's best to go sparingly on it in modern writing, as most people have been taught via sight-reading, and dialects throw them for a loop; thus they tend to lose patience with them if they're either too thick or too frequent (this is a complaint lodged these days against both Buchan and Scott, though for many years their books were best-sellers; even those with very thick dialect, such as Redgauntlet and Witch Wood).
