Is there a fool proof way to fry sausage?

If you cook the sausage low, slow, and covered (with a few table spoons of water and/or onions) until properly cooked then brown to desirable crispness you are guaranteed to be eating fully cooked sausage.

Since you love bacon, I strongly recommend getting a George Foreman grill. That way, you can cook both bacon and sausage at the same time. I'd slice the sausage to about 1/4-1/2" thick and lay them on the grill. Takes less than 10mins to cook both bacon and sausage, and you don't have to flip either. Another good thing about the grill is that the slanted surface drains the grease.

Cast iron pan dedicated to cooking meat. Once the pan is seasoned well, you need very little oil. If you fry bacon, you can simply use its oily remnants. Would recommend medium to medium-high temp. Fry until golden brown on outside and fragrant, and slightly firm. Don't be afraid to cut into one to see if the inside is finished adequately, then you will learn a timeframe that works for you.

I don't know if it's 'foolproof', but one trick I've used, when I'm not rushed, is to put some water in the pan -- you'll render off the fat in the sausage, and once the water boils off, you'll be able to brown the outside of the sausage in the rendered fat.

And yes, you can even use a stainless steel pan for this.

... and it'll take longer to cook, as you have to take the time to boil off the water. It helps to use a pan that's not much larger than what you're cooking, so you don't have to boil off lots of water.

I adopted a slow frying method recommended by some web site or other, and I've since stuck to that, except when I'm in a rush, because it's foolproof. It takes an hour, but needs very little attention during that time.

Don't prick the sausages. Fry with a tiny coating of oil, on the lowest flame you can achieve, for an hour, turning the sausages a few times.

With this method, the sausages are thoroughly cooked through. You get beautiful sticky caramelisation on the outside, and the inside remains moist and delicious.

Since the skin won't split, pretty much all the fat remains in the sausage (which is why they'll be moist) -- so this technique is best for high quality sausages.

You should be able to tell by plumpness, color, smell and if in doubt, taste.

Additionally, if you suspect they are done but aren't sure... give them a prick with a fork. If the juices run clear, you should be good to go.

I recommend a 6 dollar digital thermometer, available at your local grocer: you'll use it all the time once you have one. For the record, 140F for 10 minutes, 160F for one minutes is food safe. Pork is still going to look pink at 160F, so you're probably fine if you're at all squeamish about this.

I haven't tried it recently, but I always had problems with the sausage skin sticking to the pan. I always cook bacon on our George Forman-type grill and sometimes do sausage on it, but these days when I have sausage I usually buy a big package of sausage meat and either slice or form it into patties and fry those. The patties fry fine in a stainless pan w/ grapeseed oil.