All fats found in fresh whole foods are good, healthy, and sometimes vital. It is wise to include a full spectrum of fats in your diet, which will work hard to keep you healthy and young-looking. This, of course, is not the message we have been getting in the popular media, and because of this we have built up an unhealthy guilt complex and fear of fat.
Certain fats are indeed bad for us, but luckily it is easy to spot them. Trans fatty acids have been linked to raising "bad" cholesterol (LDL) and lowering "good" cholesterol (HDL); they are also suspected of being behind that "stubborn fat" that won't leave no matter what we do. They are the result of processing oils through hydrogenation. You can spot the presence of trans fats in a product because hydrogenated fats have to be listed in the ingredient panel of food. By adding up all the fats listed in the panel and then subtracting that number from the total listed, you will come up with the amount of trans fats in the product. However, the easiest thing to do is just avoid anything that has any hydrogenated oil in it.
Rancid fat is the other bad fat -- fat that has been mutated by oxygen, heat, moisture, and light. This fat is full of free radicals and can contribute to all the health and aging problems associated with them. At first you might think that it would be easy to avoid this one. Just reading the word "rancid" tends to make our noses wrinkle in disgust; however, we have been conditioned to accept rancid fats.
The reason that dollop of butter is salty is to help preserve it and to cover up the rancid taste. Yes, butter is good for us, but rancid butter is not. That flax oil in the refrigerator is liquid gold to some, but if it is rancid, it is worse than useless. We have to retrain our noses and taste buds to discern fresh good fats from rancid bad ones. This is one reason for using unsalted rather than salted butter.
This retraining can be done by finding stores that sell their stock quickly, store their products properly, and rotate their stock on the shelf so that no old products linger there. Also check the expiration dates on products to help you find the freshest product.
Remember that the best fats come from fresh whole foods, so try making your own butter from some fresh, pure whipping cream. Add a bit of salt to taste, if you prefer it salty, and memorize the taste to compare with commercially prepared butter. If that is a bit too much work, just find the freshest frozen unsalted butter you can, and taste that. When dealing with vegetable oils, try grinding the seeds up and then smelling the fresh aroma. Again, if this is too much of a bother, just get a good smell of the oil when you first open the bottle or tin and memorize that fresh scent. As soon as you notice that the smell is off, get rid of it.