Sausages cooked

I usually just slice down the whole length with the tip of a sharp knife, and peel the casing back in one piece.

Checking for redness is not a good indicator of doneness. For instance, freezer burned chicken tends to look less red or pink--taking on white spots and a grayish color. Some meats will also stay red no matter what. Think back to every time you saw real pork bacon. Were the meat strips ever any color except red, even when fried to a crisp?

The most advised way to check for doneness is to use a meat thermometer, inserted into the deepest part of the meat. Once it reaches a temperature you're comfortable with, that should be sufficient. The FDA publishes a table, which was revised in 2011 here. For some of their meats, I go off of other sources that actually suggest lower temperatures for more desired textures--but some people want a higher guarantee and that's fine.

For me, the main reason to heat meat is to eliminate bacteria. This chart illustrates how long it takes to sterilize cook chicken at various temperatures. The bacteria-death-rate chart will look similar for other meats, but might have different temperatures because of different natural bacterias. Often times when I cook, meats come out reddish or pinkish in the middle, and I enjoy that.

Edit: This link provides a detailed bio-chemical explanation for why meat will turn brown or not during cooking. Nitrites and nitric oxide are one explanation, but in general a lack of oxygen, presence of carbon monoxide, or certain pH levels in the meat will cause cooked-pinkness as well.

Some presence of carbon monoxide is common when cooking in a gas or charcoal oven. Furthermore, a sausage cooked in its casing fully intact will have very little oxygen available to the meat--meaning the iron inside will not oxidize and brown.