I don't buy sausages from supermarkets. I buy them, directly or indirectly, from butchers I trust (the best sausages I've ever tasted come from Woodalls of Waberthwaite, in what we are once again allowed to call Cumberland).
Either that or I make my own. I haven't for a few years now because since moving to Scotland I haven't established a source of skins, but it's easy enough and you know exactly what goes into them. You do need an old-school hand mincer with a sausage-extruding attachment (I've heard of electric mincers that will do the job but are a fraction of the fun. You need some pork shoulder with plenty of fat, some belly pork, a few anchovies, salt and pepper. Yummy!